The cabin temperature sensor in the W210 cars (at least) is a tiny fan in the same assembly as the reading light. It is similar to the cooling fan in a computer (as far as the motor is designed) and it drove me nuts on our car when I first got it. It sounded like it was coming from the dash until I leaned forward and traced it to the headliner area. It runs for about five seconds even after you shut off the car. If this is your noisy thing you can try cleaning it by vacuuming it or removing the light/sensor assembly and blowing canned air through it, mine got a lot quieter after some dust removal. HTH,
RIP "Betsy" (Oo=*=oO)
'96 E 3 2 0 (W 2 1 0) M 1 0 4 (Totalled)
Spaces in sig so as not to screw up the SEARCH; every time someone searches for that MB they don't want my sig!
2004 Audi A8L
'98 VW Passat 1.8T 5M
'87 Alfa Romeo Milano 2.5L 5M
'67 Impala convertible, 327cid