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Thumping noise revisited

I have a 2003 Honda CRV. I bought it used and at 45,000 miles, the clutch on the air conditioner compressor fell off. Apparently, this is expected behavior. Anyway, my favorite shade tree mechanic replaced the compressor–so, that is fixed.

Here’s the thing: after I got the car back, I started to hear a thumping noise coming from the glove box or footwell area on the passenger side. I would describe the noise as non-metallic–it’s as if a small person is in my glove box, knocking to get out. The noise doesn’t happen all the time–certainly not when I take it to the mechanic–but it only occurs when the air conditioner is on. Also, the noise has a kind of weird timing. It seems to be related to the car shifting into 2nd. If the thumps were music, there would be one half note, followed by two quarter notes. That is the general pattern, but there is some variation.

My mechanic and one of my friends drove a half a tank of gas out of the car trying to hear the noise. They put it on the rack, and never heard anything. I drove it to another town last week and it never made he noise, but today it made it repeatedly.

Is this familiar to anyone? What is happening?!

Inside of the heating/cooling system are little doors that open and close to direct air around - one for fresh air from the outside, one for blending for temperature, etc. These are often vacuum controlled, or by little electric motors. It sounds like one of yours is intermittently malfunctioning. They do go thump if they aren’t being controlled exactly correctly, and sounding like a little person trying to escape the glove box is probably a pretty good description of what this would sound like.

People ask “what could this be” but they don’t ask “after my car has been thoroughly inspected and no problem found can I live with a unexplained noise from my car”, the answer, sure you can.