Something else it may be... The exhaust has 2 hangers connected to the transmission crossmember. There is a Ford Service Bulletin noting how the hanger pegs (attached to exhaust) can rattle off the crossmember. Ford says to remove the black rubber hanger from the crossmember and bend the hanging pegs down away from the crossmember as they aren't needed.... they are there to help with assembly at the factory.
1966 Mustang Sprint 200--Wimbledon White
1998 Mustang V6 Coupe--Silver--RIP, Totalled by brother
2005 Mustang V6 Premium Coupe--Sonic Blue