Not to dismiss the fuel pumps, but a leak in the locking system can cycle the vacuum pump on and off. The vacuum pump will also sounds a little like a bee buzzing when it is cycling.
'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.