My '95 C280 had groaning noises periodically from the aux. vac pump located in the rear passenger side.
That lasted months and stopped only to develop a high pitch 'whistle' from the passenger side front area toward the right.
It is periodic and can be made to stop by pressing the EC (economy) air setting or any other climate setting.
All the vents work properly and the rear head restraints drop when the dash button is depressed. (supposedly this is a test for vacuum).

Apparently the recirculation/fresh air pod (element) has a tendency to leak and this may be the source of the 'whistle' noise.

Does the dash really need to be removed to access this part? I believe it sits behind the glove box air bag. (?)

Is there any liquid spray that can be injected to stop a pin hole leak in the diaphragm, assuming that is the problem??

Does the dash come off as one piece or is it compartmentalized?

Your opinions are appreciated. I did not find answers in the archives.

Thanks.

__________________
1979 300D 220 K miles
1995 C280 109 K miles
1992 Cadillac Eldorado Touring Coupe 57K miles SOLD
********************
1979 240D 140Kmiles (bought for parents) *SOLD.
SAN FRANCISCO/(*San Diego)
1989 300SE 148 K miles *SOLD