Sounds like the main flap slamming open/shut as soon as it receives vacuum from the now started engine. The 560 does that if I have the ACC set to off, it slams with a solid thud on start-up.
'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.