A bad mount could be causing the fan to eat at the shroud. I had one break on the left side of a 440, everytime you backed up there was a terrible grinding noise. It went away when the shroud did. Ended up replacing the mount and the shroud. Check the condition of the upper inside rim of the shroud.
Might also be a sticking bendix, keeping the star drive engaged, or partially engage to the ring gear.
'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.