30 or 40 Popping sounds under drivers dash

2003 Tahoe, original owner. About 6 weeks ago was driving when Battery light came on. I watched the voltage meter slowly go down, so figured the alternator just died. I shut off everything electrical I could think of and drove straight to an NTB. Got there just as the car was shutting down the dashboard and starting to sputter. They tested the alternator and sure enough, dead. Easy replacement, and back on the road with same battery.

At that point, every time I turned on the truck, after about 10 or 15 seconds, a popping noise began to come from under the driver's side dash, to the right of my right leg. It sounds like a plastic-y pop of maybe a gear being pushed too far, or a servo not pushing a door past something... there's a very faint whining/servo sound preceding it and after it stops.

Right after I first got the alternator replaced, it would pop every time it started up, would pop 30 to 40 times, and then stop. Then there's a little servo sound and then nothing. I notice the air blows more forcefully through the upper vents on the driver's side just preceding the popping, and then while it's popping, and then it goes back down to the foot vents after it stops. I've also noticed that if the HVAC system is powered OFF at startup, it only pops once or twice, or sometimes not at all.

Heat and cool work normally otherwise.

After about a week of this, the popping stopped going 30 or 40 times and started to just pop between 0 and 4 times. While I was ready to take it to the dealer for the 30 pops, this was manageable and I was just going to live with it. I figure it is something to do with the HVAC and otherwise that seems to be working, so let it go.

A couple of days ago, my battery died. Came back after a baseball game and the battery was dead enough that it would not crank, and all the electrical stuff zeroed out. Got a jump, started back up fine. Ran it home, and let it idle for 20 minutes outside. Next morning it cranked slowly but started. But after driving for only about 10 minutes and leaving it, it would again not crank. Got a second jump, drove it for about an hour before shutting off. About 2 hours later, it again would not crank. So thinking dead battery I went to the Costco for a brand new one to replace my battery.

Now the car starts up reliably again, but the popping sound is once again up to 35 to 45 pops. Again, it's too annoying to leave, but I'm at a loss to identify what it is.

I took the lower dash plate off, and am able to hear that there is the whine of a servo just inside the middle dash/bulkhead. I haven't taken more of the lower dash apart yet, but am hoping someone on here might have experienced what this is before. I'm okay replacing parts myself, but the repair shop wants almost $200 just to diagnose this and then I'm thinking I could be in for a $300-500 part, so am trying to keep costs down on my 12 year old friend.

Sorry for the lengthy post, but there are a lot of details leading up to this that might be useful for a car detective I hope. My rear heat also doesn't work, though rear AC works fine, if that might have anything to do with it. That has been the case for about 4 years though, and so far having the kids just bundle up when they ride with dad has addressed that problem...

Thanks for any help.

KEY OFF , disconnect the battery ...negative connection , then turn key to on .. allow 15 min to reset the computer .. key off reconnect battery and hopefully the hvac computer will reset.

Thanks jCAT, I tried this twice, once for about 15 minutes, and once for about an hour with the battery disconnected. There was no change in the behavior.

also one change from my original post. I said it didn't pop if you turned the car on with the HVAC system off, that's not so. Even if everything is off at ignition, the popping happens on startup.

Any other ideas?

Thanks

Thanks jCAT, I tried this twice, once for about 15 minutes, and once for about an hour with the battery disconnected. There was no change in the behavior.

also one change from my original post. I said it didn't pop if you turned the car on with the HVAC system off, that's not so. Even if everything is off at ignition, the popping happens on startup.

Any other ideas?

Thanks

One of the blend door actuators is defective (popping sounds indicative of this) and is causing battery to drain while shutdown due either to being out of position and attempting to return to base position or internal short in the servo.

This is a cheap part and is normally fairly easy to replace. However, the driver's side actuator (which it sounds like in your case) removal/installation I believe is somewhat more involved. Search for youtube videos, should be plenty to help you along.