howdy,
Can you actually feel a vibration, or just hear a rattle? If it’s just a rattling sound, it’s a very good chance that it is your cat heat shield, because it seems nearly every ST170/SVT owner on earth has had the same problem (including me). I have a fairly audible rattle on my car, especially when taking off, can't actually feel a vibration though.
Some guys have taken their ST's back to Ford 3 or 4 times to have the heat shield clamps retightened, but the shield simply works its way loose again. Apparently if you ignore it long enough, it will eventually fall off. So my solution for the moment is to turn my stereo up loud and pretend it isn't there until it drops off
: . Seriously though, let me know if you have any success fixing it yourself, and I'll have a go at mine too.