Strange Hissing Noise - '99 A4 1.8T

I hear a hissing noise when I enter the car; it comes from the rear passenger side. Lasts a couple of seconds, then goes away.
(I had an accident couple of years back; the rear passenger side quarter
panel was hit pretty bad and was rebuilt. Looks good as new now, but on close inspection the door lock on the rear passenger door seats slightly lower into the door than do all the other door locks when they're locked, not quite as flush with the door as the others.)
Am I just hearing the fuel pump when I get in? Or does Audi use compressed air to control the door locking mechanisms? I'm wondering if I've got an air leak that will eventually cost me a bundle when/if it no longer locks/unlocks my doors.
Any ideas? Or am I just an auto-hypochondriac?
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wrote:

The central locking system is pneumatic. If you can hear it, you can find where the sound is coming from, and the pump is located in the right-rear corner of the boot, in the floor.
/daytripper '00 s4 6spd
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