2020 Nissan Maxima Knocking Noise: What's Wrong and How to Fix It

Overview

That persistent knocking noise from your car can be more than just annoying—it’s a direct message from your vehicle that something isn’t right. Based on analysis of 100 owner discussions, a knocking sound is a common complaint that can stem from a surprisingly wide range of issues, from a simple loose plastic shield to serious internal engine problems. The key is to not ignore it. This guide will walk you through the most common culprits and fixes, as reported by fellow car owners who’ve been in your shoes.

How It Manifests

Owners describe this problem in a few consistent ways. The knocking is often rhythmic, speeding up as you accelerate and sometimes disappearing at certain RPMs. Many report it’s most noticeable at idle or low speeds, coming from the front of the vehicle. A frequent description is a sound like a woodpecker or someone tapping rapidly on metal. Crucially, in several discussions, this noise was accompanied by check engine lights, with codes like P0011, P0021 (related to camshaft timing), and P0174 (system too lean) popping up, which are vital clues.

The Usual Suspects

Based on confirmed fixes and repeated mentions across the forums, here are the most common sources of a knocking noise, ranked from most frequent to least:

1. Loose or Broken Plastic Undershields/Aero Panels: This was the #1 fix in numerous threads. A plastic piece under the engine bay can crack or have its fasteners break, letting it flap or tap against the frame or ground.
2. Accessory Drive Belt & Components: A worn serpentine belt, a failing tensioner, or a bad pulley (like the idler or alternator) can create a loud knocking or slapping sound, especially at idle.
3. Exhaust System Contact: The midpipe or another part of the exhaust can come loose from its hangers or develop a crack, allowing it to knock against the underbody or crossmember.
4. Engine-Related Issues (More Serious): While less common as a first guess, codes like P0011/P0021 point toward variable valve timing (VVT) solenoid issues or low oil pressure, which can cause a valvetrain knock. A vacuum leak (associated with P0174) from a cracked vac line can cause a lean condition and irregular running that may sound like a knock.
5. Loose Body Trim or Parts: Items like clear corners (aftermarket lights) or other non-engine components were mentioned as occasionally being the source of a baffling rattle or knock.

Finding the Problem

Diagnosis starts with simple observations. Here’s a methodical approach used successfully by forum members:

• Listen and Locate: Have a helper gently rev the engine while you listen around the front and under the car. Does the knock change with engine speed (points to engine/accessory drive) or with road bumps (points to exhaust or body parts)?
• The Visual and Physical Inspection: This solved most cases. Get under the car (safely, on jack stands) and physically push, pull, and wiggle the plastic undershields. Look for cracks or missing clips. Visually trace the exhaust, especially the midpipe, looking for shiny contact marks where it might be hitting.
• Check the Accessory Drive: With the engine OFF, use a long socket extension as a stethoscope. Place one end on the bolt in the center of each pulley (idler, tensioner, alternator) and the other near your ear. When you start the engine, a bad bearing will transmit a loud grinding or knocking sound directly to your ear. Never place anything near moving belts with the engine running.
• Scan for Codes: If your check engine light is on, pulling codes is non-negotiable. Codes like P0011 or P0021 direct you to the engine's top end, while P0174 suggests checking for vacuum leaks.

How to Fix It

Start with the simple, cheap fixes first. This is the order of success rates from the discussions analyzed.

1. Secure or Remove Loose Plastic Shields (Most Common Fix): If you find a cracked or flapping plastic piece, you have options. You can often secure it temporarily with heavy-duty zip ties. For a permanent fix, replace the broken clips or the entire shield panel. This is a 15-minute, sub-$20 DIY fix in most cases.
2. Replace the Accessory Drive Belt and Tensioner/Pulleys: If your noise is from the belt area, replace the belt. If the noise persists, the tensioner or an idler pulley is likely the culprit. Replacing the belt and one pulley is a very common mid-level DIY job.
3. Reposition or Repair the Exhaust: If the midpipe is knocking, check its hangers. They can be re-seated or replaced. If it's cracked at a flange, it may need welding. A shop can often bend a hanger slightly to gain clearance.
4. Address Engine Codes (Professional Help Recommended): For codes P0011/P0021, the fix often involves replacing VVT solenoids or checking oil flow. For code P0174, you need to hunt down and replace any cracked vac lines. These require more mechanical knowledge. If you hear a deep, heavy knock from inside the engine itself (likely not the issue you're reading about here), stop driving and consult a professional immediately.

What to Expect to Pay

Costs vary wildly based on the cause and who does the work.

• DIY on Simple Fixes:

  • Plastic shield clips or zip ties: $5 - $20
  • New accessory drive belt: $30 - $80
  • Idler or tensioner pulley: $40 - $150 (part only)

• Professional Shop Repairs:

  • Accessory belt & one pulley replacement: $200 - $400
  • Exhaust hanger repair/replacement: $100 - $250
  • Diagnosing and replacing VVT solenoids (for P0011/P0021): $300 - $600+
  • Diagnosing and repairing a vacuum leak (for P0174): $150 - $300

The lesson here is that a quick DIY inspection can save you hundreds by identifying a simple loose panel before you pay for diagnostic time at a shop.

Avoiding Future Issues

Prevention is about periodic checks. Every few months or before a long trip, take a quick look under your car. Give plastic undershields a gentle push to see if they’re still secure. Listen for any new sounds during the first minute of a cold start. Adhere to your vehicle’s recommended service intervals for belt replacement and, most importantly, use the correct grade of oil and change it regularly. Proper oil maintenance is the best way to prevent the expensive internal issues that cause knocking.

Source Summary: This analysis is based on 100 owner discussions from Reddit and automotive forums.