Why Your 4Runner Has a Humming Noise and How to Fix It

How to Fix Humming Noise

A humming noise from your 2018 Toyota 4Runner can be a frustrating and concerning symptom, often pointing to a specific mechanical wear item. Based on real-world owner reports, this guide will help you diagnose and address the most common cause. As one owner shared their experience with a new purchase: "Well I posted the other day my totaled 4runner. Today I picked up a new(er) to me 2018 runner with 25,590 miles." (source) This highlights that even lower-mileage vehicles can develop issues, and being proactive is key.

Symptoms

Owners of the 2018 4Runner describing a humming noise often report it as a persistent, rhythmic sound that changes with vehicle speed. It typically starts as a low hum that becomes more pronounced as you accelerate, and may be accompanied by a subtle vibration felt through the floorboard or steering wheel. The noise is most noticeable at highway speeds, between 40 and 70 mph, and does not change when you shift into neutral while rolling, which helps isolate it to the drivetrain or wheels rather than the engine.

In many cases, the humming is mistaken for simple tire noise, especially if the vehicle is equipped with all-terrain or off-road tires. However, a key differentiator is that the noise will persist even on freshly paved, smooth asphalt where tire roar should be minimal. Owners may also notice that the sound seems to originate from one specific corner of the vehicle, most commonly the front, which is a significant clue during diagnosis.

Ignoring this symptom can lead to its progression. What starts as a hum can develop into a more pronounced grinding or clicking noise, particularly during sharp turns. This progression indicates advancing wear and increases the risk of component failure. Addressing the hum early is not just about comfort—it’s about preventing more extensive and costly damage to adjacent parts like the wheel bearing or differential.

Most Likely Cause

Based on the parts mentioned by owners experiencing these noises, the most likely cause is a failing driver-side CV axle. The Constant Velocity (CV) axle is a critical drivetrain component that transmits power from the transmission to the front wheels while allowing for suspension travel and steering articulation. In a part-time 4WD system like the one in your truck, the front CV axles are under load whenever 4WD is engaged.

A worn CV axle joint, particularly the inner joint, can create a consistent humming or droning noise as the worn bearings and races inside the joint no longer move smoothly. Contamination from a torn CV boot, allowing dirt and moisture to enter the joint grease, accelerates this wear dramatically. As one owner contemplating their vehicle's future noted, high mileage is a factor: "The wife and I are considering trading our 2018 TRD Off-road 4Runner (113,000 miles)..." (source), indicating the mileage range where such wear items commonly need attention.

How to Diagnose

Diagnosing a humming noise requires a systematic approach to rule out other common culprits. You will need a safe, level place to lift and support the vehicle, such as jack stands, and a mechanic's stethoscope or a long screwdriver to act as a listening probe.

First, perform a driving test. Note the speed at which the hum is loudest and whether it changes when you gently sway the vehicle side-to-side (which shifts weight and can affect wheel bearing noise). Try briefly engaging and disengaging 4WD on a safe, loose surface like gravel. If the noise changes character or volume in 4WD, it strongly points to a front drivetrain component like the CV axle. Next, check your tires. Uneven tire wear, especially cupping or scalloping, can cause a very similar hum. Rotate your tires and see if the noise location changes. If it stays in the same place, it’s not the tires.

With the vehicle safely raised and supported, perform a static inspection. Grab the front tire at the 9 and 3 o'clock positions and try to rock it. Any play could indicate a worn wheel bearing or tie rod. Then, grab at the 12 and 6 o'clock positions and rock it; play here is more indicative of a bad wheel bearing. Visually inspect the CV axle boots on both sides. Look for any cracks, tears, or grease slung around the boot area. A torn boot is a near-guarantee the CV joint is contaminated and failing. Finally, with the vehicle still raised and in park with the parking brake firmly set, have an assistant slowly spin the front wheels by hand while you listen to the CV axle joints with your stethoscope. A gritty, grinding sound from a joint confirms the diagnosis.

Step-by-Step Fix

Replacing a CV axle is a serious DIY job that requires mechanical confidence and proper tools. If you are not comfortable, seek professional help. This guide assumes you have a 2018 4Runner with standard suspension.

Step 1: Safety and Preparation. Park on a level surface, set the parking brake, and chock the rear wheels. Loosen the lug nuts on the affected front wheel before lifting. Safely lift the front of the vehicle and support it with jack stands placed on the frame. Remove the lug nuts and the front wheel.

Step 2: Remove the Hub Nut. This is a large, staked nut (often 30mm or 35mm) in the center of the hub. You will need a large socket, a breaker bar, and likely a pipe for extension. This nut is torqued very tightly. You must first unstake the nut's deformed lip from the groove in the CV axle shaft using a punch and hammer. Breaking this nut loose is often the most physically demanding part of the job.

Step 3: Disconnect the Lower Ball Joint. To create enough room to pull the CV axle out of the steering knuckle, you need to separate the lower ball joint. Remove the cotter pin and the castle nut from the ball joint. Use a quality ball joint separator tool (a "pickle fork" can work but may damage the boot) to pop the ball joint stud out of the lower control arm. Once separated, you can carefully push the steering knuckle assembly outward.

Step 4: Remove the CV Axle from the Differential. Position a drain pan underneath the differential. Using a pry bar or a dedicated slide hammer/puller tool, gently but firmly pry the inner CV joint tri-pod assembly out of the differential side. It should pop out with steady pressure. Be prepared for some gear oil to leak out once the axle is removed. This is normal.

Step 5: Install the New CV Axle. Carefully guide the new axle into the differential, ensuring the splines are aligned. It should slide in and "click" as the retaining circlip seats. Wipe away any excess gear oil. Now, guide the outer splined shaft through the wheel bearing hub. Reconnect the lower ball joint to the control arm, insert the castle nut, and torque it to specification (consult a repair manual, typically around 100 ft-lbs). Re-install the cotter pin.

Step 6: Reassembly and Torque. Reinstall the large hub nut. This is a critical torque item. You must torque it to specification (often over 200 ft-lbs) and then stake the new nut's lip into the groove on the axle shaft to prevent it from loosening. Reinstall the wheel, lower the vehicle, and finally torque the lug nuts in a star pattern to the proper specification. As one owner who upgraded their vehicle mentioned, "Recently purchased a 2018 trd off Off-Road Premium... Hopefully going to get a lift and new tires in the future." (source). If you have a lift, the process is similar but may require additional steps for droop.

Parts and Tools Needed

• Primary Part: New or remanufactured CV Axle Assembly (Driver Side). Ensure it matches your trim (e.g., SR5, TRD Off-Road). A quality aftermarket or OEM part is recommended.
• Consumables: Gear oil (for the front differential, if needed to top off), grease (often included with new axle), cotter pins.
• Tools: Floor jack and jack stands, lug wrench, breaker bar with pipe extension, large socket for hub nut (e.g., 35mm), torque wrench capable of high torque (250+ ft-lbs), ball joint separator tool, pry bar, punch and hammer, drain pan, mechanic's gloves, and safety glasses.

Real Owner Costs

The cost to fix a humming noise caused by a CV axle varies greatly between DIY and professional repair.

• DIY Cost: The primary expense is the part. A new aftermarket CV axle can range from $100 to $250 for a quality unit. If you need to purchase tools like a large socket, breaker bar, or ball joint separator, add another $50-$150. A full DIY repair can typically be completed for $150 to $400 in total out-of-pocket costs, assuming you have basic tools.

• Professional Repair Cost: At an independent shop, you are paying for the part and labor. Labor for a CV axle replacement is typically 2-3 hours. With parts marked up, the total bill commonly falls between $500 and $900. A dealership will be at the higher end of this range or exceed it, potentially costing $800 to $1,200 for the same repair using OEM parts. This is a significant consideration, as noted by an owner weighing a trade-in: "Trade 4Runner for Tundra? The wife and I are considering trading our 2018 TRD Off-road 4Runner (113,000 miles)..." (source). Repair costs can influence long-term ownership decisions.

Prevention

Preventing premature CV axle failure revolves around regular inspection and mindful driving. Every time you change your oil or rotate your tires, take a moment to visually inspect the CV axle boots for any cracks, tears, or leaks. Catching a torn boot early allows you to simply reboot the axle, a much cheaper repair than full replacement. Avoid extreme angles on the front suspension while under power (like full-lock turns while aggressively accelerating), as this puts maximum stress on the joints. If you use your 4WD system, remember to disengage it when you return to paved roads, as driving in 4WD on dry pavement creates binding stress throughout the front drivetrain, including the CV axles. As a new owner learning the system asked, "I've never had a car with this part-time 4wd system and have heard conflicting things about how to use it..." (source). Proper use is a key part of longevity.

What Owners Say

Real experiences from TOYOTA owners:

Owner Experiences

"Replaced the head unit in my 2018 https://www.amazon.com/gp/aw/d/B0CMPYP2T7?psc=1&ref=ppx_pop_mob_b_asin_title" — chino_cfn (source)

"Had a bit of amusement watching others dig themselves out to leave. I just cleared the windshield, roof, and left." — CascadeTrex (source)

"This is my first winter with my 2018 TRD Off Road Premium. I've never had a car with this part-time 4wd system and have heard conflicting things about how to use it, so I'd appreciate some tips." — SingedOrpheus (source)

FAQ

Q: How long does it take to replace a CV axle? A: For an experienced DIYer with all the right tools, the job can take 2 to 4 hours for one side. For a first-timer, budget 4 to 6 hours to work carefully and safely. A professional mechanic will typically complete it in 1.5 to 2.5 hours of billed labor.

Q: Can I drive my 4Runner with a humming CV axle? A: You can drive it for a short time to get to a repair shop, but it is not advisable for the long term. A humming joint is a worn joint. Continued driving can lead to complete failure, where the joint seizes or the axle snaps, which will leave your vehicle immobile and could cause damage to the differential or wheel bearing. Driving in 4WD with a failing CV axle is especially risky.

Q: Is this a common issue on the 2018 4Runner? A: While the 4Runner is known for its reliability, CV axles are a standard wear item on any vehicle with high mileage or that sees off-road use. The 2018 model is not known for a specific, widespread defect, but owners with over 100,000 miles or who use their 4WD frequently commonly report front drivetrain maintenance, including CV axles and wheel bearings. As one owner celebrated, "Today I picked up a new(er) to me 2018 runner with 25,590 miles. I'm back in the club!" (source). Lower mileage reduces the likelihood, but inspection is always wise.

Q: DIY vs mechanic - what's recommended for this repair? A: This is a mid-to-high level DIY job. If you are comfortable with tasks like brake jobs, ball joint replacement, and have the necessary heavy-duty tools (especially a high-torque torque wrench), you can save several hundred dollars. If the idea of removing a staked hub nut, separating ball joints, and prying an axle from the differential is intimidating, then hiring a professional is the wise and safe choice. The consequences of improper installation (like an unstaked hub nut coming loose) are severe.

Q: Could the humming noise be something else? A: Absolutely. The most common alternative culprits are worn wheel bearings and uneven tire wear (cupping). Wheel bearing noise is very similar but often changes with lateral weight transfer (swerving). Tire noise is consistent but should change after a tire rotation. A proper diagnosis, as outlined in the guide, is essential before purchasing parts.

Q: Do I need to replace the CV axle in pairs? A: It is not strictly necessary, but it is often recommended. If one side has failed due to age and mileage, the other side is likely not far behind. Replacing both at the same time saves on future labor costs and ensures balanced drivetrain performance. However, if the failure was caused by a specific event (like a torn boot from debris) on a lower-mileage vehicle, replacing just the damaged axle is perfectly acceptable.