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The knock, knock, knock sound from the GMC Sierra

Call 2

Debra Bellingham, Washington - 1994 GMC Sierra 1500

Debra’s hearing a mysterious knocking sound–it happens at random times, sometimes when the AC is on, sometimes before she turns the key, and sometimes after she turns the key off. It seems to be coming from behind or below the glove compartment–there’s a little box below the glove compartment. Tom and Ray are fairly stumped–it may be something caught in a fan, like a twig. It sounds like a power antenna motor, except the car doesn’t have one!

Come on, guys, I know you’ve answered a similar question on a different vehicle before. This is the air diverter door for the AC. It’s probably controlled by an electric motor, and the limit switch is bad. It’s made to default to the defrost position when you turn off the key. When it moves to the defrost position it’s supposed to stop when it hits the limit switch. If that switch is bad, it will keep driving the motor and a ratchet or knocking sound is made until it times out. A twig or pine needle striking the fan blades sounds totally different and would vary in frequency with the blower speed.

This same problem has come up before! I was listening to a few of the archived shows from March 2008, and lo and behold in show #812, a caller with a 1994 GMC Sierra had the exact same problem. Back then, Tom and Ray diagnosed the noise as being caused by a bad motor on the air diverter door, but they called it a flap or flapper. Sounds like a fairly common problem on these trucks.

I think you’re correct. Tom and Ray usually get the easy ones right…somehow they blew this one.

Sounds to me like the blend door or recirculator door motor is popping as it tries to move the door.

Anybody who has spent any amount of time working on these trucks ('88-'94 GM 1/2, 3/4, and one ton) knows about that blend door motor that strips out the plastic gears and makes that knocking sound. I would be astonished if Tom and Ray had never seen or heard of this issue before, but they have messed up on some pretty simple stuff before.

I was listening to the show online and I was going crazy. I have dealt with the noise numerous times and I agree with mark9207. The blend door motor gears strip and make that lovely thump noise. It’s pretty common.

As Junebug mentioned, they DO know about blend doors flopping around, and have correctly diagnosed this on past shows. They just had a spell of brain lock this time…

…maybe that’s why they want to retire!

Like asilver I too was yelling at my monitor over this diagnosis. My 94 Suburban does the same thing but only when it’s cold and I’m using the defrost setting (my ac gave up the ghost years ago). I have to say the first time this happened was really disconcerting because the damn thing seemed to knock on forever after I removed the key (it stopped when I gave it a good kick). My half***d solution has been to shut off the defroster before I shut off the truck. Seems to have ‘cured’ the annoying knocky knocky run-on.

Yes, the reason it rang a bell with me was I had a '96 Nissan Quest with the rear A/C. I would hear the clicking back there every time I started the engine. It was always the same number of clicks (I think 18) before it timed out. There was a diverter door that would direct the air to either the upper A/C vents or the heater vents down by the floor. The limit switch had failed and looking at the manual it would have been a real bear to get to. So I “fixed” it by cutting a wire so that it stayed on the upper vents since we are in the Deep South and use the A/C 95% of the time.

I also was surprised the guys didn’t mention a possible cause of the knocking noise being the vacuum and/or electrical controlled heating/cooling louvres going spazzy once in a while. They’ve mentioned this several times before as a cause of this kind of noise. When I heard the call I was thinking to myself that maybe this particular vehicle the caller was calling about didn’t have that type of ventilation system, so the guys knew the noise had to be something else.