It could be the exhaust shields, I'm pretty sure the exhaust shields are the cause of the noise on my car. Or, and this is a big or.... there was a technical service bulletin about it... if you look under your car, the exhaust y pipe is connected to the transmission crossmember on both sides with hangers... The service bulletin says to remove the black rubber hangers and bend the pegs on the exhaust down away from the crossmember, as they were only there to aid in 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