It is the defrost/vent diverter flapper. If it closes too far it will jam in the duct and make a loud pop when it pulls loose. It is controlled by a long rod that goes up over the center hump by the gas pedal. I shortened it slightly (is adjustable) and it stopped popping.

Can test by going to DEFROST , wait a few seconds, and then back to VENT and it should pop again.

This pop you hear. Is it the same pop that sounds like a bunch of coins being dropped in a plastic bin? It almost sounds like chattering, or somehting like that.


Is it caused by that rod, under the gas pedal? Hmm, I should take a look tommorrow.

Tim,
Its more like one loud POP. Like popping a plastic bag.

I noticed on a cool morning one day that if I started the car with the defroster instead of the heater on, there is no popping noise.

Hmm thats interesting- mine has been doing this for a while now- only when its really cold out tho. So adjusting the rod will releive the problem?

The problem is trying to get up there with your hand and adjusting the rod shorter. I still need to do mine. PITA

fair enough. I seem to recall seeing that rod when I was doin some work to the car the other day. Maybe I'll try tonight. Thanks