Just bought a mustang. Need help!!!

Could be so many things, to check trans mounts use a prybar.

Yeah.... that could be anything. I'd put it up on stands and check it out. If you can't see anything obvious then take it to a shop and have them look at it so you at least know what's going on.

I'm bettin' on u-joints...is there a pretty good vibration when starting out?

Clunking from a Mustang drivetrain? Say it isn't so...

When are you hearing this noise? Acceleration? Decel? Transition between coast and drive/overrun? When the trans shifts?

I dunno. My 5-speed GT makes some "high quality" sounds from the driveline and it's done it basically since new. As I back the car into the garage and an feathering the clutch, I can easily get clunks that would do a dump truck proud depending on how I load/unload the thing. Sometimes when I shift to first from neutral at a light I'll hear similar things. I gave up worrying about it long ago. I've been under the thing and checked everything and all is good and tight.

In my case, I think it's just tolerance stack from the fit of the clutch disc on the input shaft splines all the way back to the tolerance between the axles splines and the side-gears. Your auto won't be a whole lot different.

Still, get the U-joints and mounts looked at just in case. The car is 8 years old now and it probably wasn't babied all those years so you never know.