Are you sure it isn't one of the vacuum operated flaps in the dash slamming shut once the vacuum is gone (from shutting down the engine) due to a large vacuum leak? Sound might be reverberating off the windshield, through the defroster vents, could make it sound as if from above.
'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.