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Knocking Sound

I have a loud knocking sound coming from under the right side dash area when I shut off the motor in my 2006 Chevy Impala LT. The knocking sound last for about 30 seconds, and then stops.

Do you have any ideas what this might be?

Could be the heater core.
Have you checked the coolant level lately?


Thanks! I’ll have it looked at tomorrow.


This is a known issue with a “blend door” in the HVAC system on GM vehicles of that vintage. This little flap is not closing or opening as it was designed to do, and it is continually going through its cycle of attempting to do what it is supposed to do.

If this is electrically-controlled, as I think it is, then either the linkage from its little electric motor is broken or the little motor itself needs to be replaced. If this is vacuum controlled (probably not), then it is possible that you have a problem with the servo motor.

Thanks for taking the time to help. Much aprreciated.