Envoy with "Door Knocker"


I have a 2002 GMC Envoy with about 99000 miles. It has started to have a loud clacking or knocking sound that happens 4 times in rapid succession every time I start the engine. It only does this once.

If it happens to not occur then it will sometime during the driving session, but again, just one episode.

It is quite loud and appears to come from the center of the dashboard under the windshield. It sounds like it’s in the passenger compartment, not in the engine room.

Any thoughts?



I strongly suspect that one of the “blend doors” or some other moveable part in your HVAC system is flapping. Have the HVAC system checked, and I think that you will find the problem.


That’s what my wife and I have been suspecting that it’s in HVAC due to the location.



More specific, there is probably a vacuum leak that allows the doors to move to their resting position some time after the engine is shut off. Does it do this even with an immediate restart? It could be a bad check valve between the vacuum source and the reservoir. They are usually pretty easy to find and replace.


I’m pretty sure these are controlled by stepper motors, not vacuum controlled. There is a known problem with mode select doors on the TrailVoy platform. You may have better luck asking on http://forums.trailvoy.com/index.php