Duramax Diesel Warm Engine Idle Knock Sound

Some 2020-2021 Silverado 2500HD/3500HD and Sierra 2500HD/3500HD trucks equipped with the 6.6L Duramax diesel engine (RPO L5P) may have a thumping or knocking sound coming from the rear of the engine. The sound is often more pronounced in the interior of the truck as it’s amplified by the transmission bell housing.

To duplicate the engine sound:

  • Idle the engine at about 600 RPM with the transmission in Park or Neutral. The engine oil temperature should be 113–185°F (45–85°C).
  • Turn the HVAC fan and radio off.
  • Remove the accessory drive auxiliary belt. (Fig. 14)

If the noise level is significantly reduced, it should be considered a normal characteristic. No additional repairs are necessary.

Fig. 14

Use the following procedure for accessory drive auxiliary belt removal, testing and installation:

  1. Remove the air cleaner housing.
  2. Remove intake air duct.
  3. Separate the charge air cooler (CAC) tube and position aside.
  4. Remove the belt and position aside.
  5. Install the CAC tube.
  6. Loosely place the air box and intake duct back in place. (So a DTC is not set for the sensor when starting.)
  7. Start the engine and verify the sound is gone.
  8. Remove the loosely installed air box and intake duct.
  9. Separate the CAC tube.
  10. Install the belt.
  11. Install the CAC tube.
  12. Install the air box and duct.

TIP: Do not run the vehicle more than three minutes without the auxiliary belt installed. Do not raise the RPM higher than idle with the belt removed.

The engine sound has no short- or long-term effects on the engine.

– Thanks to John Stempnik


Updated November 30, 2020

Poor Heater Performance Due to Coolant Hose Restriction
Unable to Deactivate Brake Boost System


  1. would this also apply to the 2019 without the auxiliary belt? And if so is there a doc id that we can use to stand behind?

  2. So NPF warranty claim then? Hmmm

  3. Bowen Ferre says:

    This Drives people crazy that are used to listening to the beautiful Duramax sound from previous years

Leave a Reply

Your email address will not be published. Required fields are marked *