TwoSeven

And definitely NOT suspension-related. When I turn the key to the ON position (with the engine OFF) after the car is sitting for a while (the noise is louder when the car is truly cold) the car makes a noise, which seems to be climate-control related, that sounds almost like a quiet machine gun. The sound is like a repetitive popping sound, maybe 20 pops, and it sounds like it is coming from above the passenger footwell. (The car, by the way, is an 05 S4 cab.) I can make the noise happen, albeit much quieter and a few less pops, if the car is a bit warmer, but it requires turning it off, opening the door and closing it, locking and then unlocking it, then closing the door and returning the key to the ON position, again WITHOUT starting the engine. With the car fully warmed, no sound at all.

It is in for service in the morning - any tips on what to ask about? The car sat for three days recently, the longest since I got it, and it sounded VERY loud...

ruknd

Maybe its your tensioner building tension?

SharkyS4

tip or manual? if manual could be a clutch bearing

TwoSeven

The key is in the ON position, engine OFF in this scenario. As in, before starting the engine. So it isn't tensioner slack or really in any way drivetrain related. Something electronic or controlled thusly is causing the sound.

TwoSeven

Fuel pump priming the rail. Will be replaced ASAP!