How about front wheel bearings? They can have a "rattling chain" sound. That came to mind after your comment about hearing it from the outside when crossing a bridge. Might be worth a shot.
'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.