Had this problem ever since I bought mine. It only happens once or twice a month and it doesn't seem to effect the operation at all. Its the upgraded Pioneer six disk in dash.
At first I thought it had something to do with the fact that I leave my radio on and it happened when I turned the key on to start the car. So, I started turning my radio off before I shut down the car, but it still happens when I turn the radio on.
The volume of the radio doesnt seem to have anything to do with the volume of the pop.

Other than scaring the heck out of me every month or so, there doesnt seem to be any other problem, and the dealer could never track it down when it was under warranty because it, of coarse, would never happen when the car was in their shop.
Anyone else have this problem, and if so, what was the fix?

 

There's a TSB on this. Search.

The fix is to reprogram the BCM.

Mine had it and of course when I took it to the dealer they said they couldn't recreate the problem. I had a copy of the TSB with me. I told them I can't waste my time messing with this. Just note that you "DID" experience the problem and reprogram the BCM already! They did and it never happened again! Good luck!