Sounds like a vacuum leak. Start by checking all rubber connectors to the vac lines under the hood, replace any that are hard, or better yet just replace them all. Check for any loose vac lines and reconnect. Hopefully it won't be one of the vacuum acuators for the climate control, which are located in the dash.
Don't overlook the vacuum line to the modulator on the tranny, it may be off, or leaking too.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.