T5 Vibration- New to tranny

I have had mostly autos, and I just got a T5 that I have not tore into yet. The problem is, the person I got it from said that at highway speeds, it had vibration, so assuming that this is only while it is in gear, and goes away in neutral, and is actually something in the tranny, what could it be?

And just for fun, other than driveshaft, what else could cause this, flywheel? Clutch? Etc.

Thanks for the help,
Brian

Well, this is good, as most personal experiences seem to be not related to the tranny itself, and everything else is getting replaced.