You didn't specify when the noise is happening.

On my 500SL, one of the vacuum pods for the HVAC system leaks. Sounds like someone letting air out of a balloon! A/C doesn't work anyway, so I have that vacuum line blocked off.

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]