Id put my money on any of the exhaust heat sheilds bolted to the manifolds etc, either that or maybe your IC...
I dont know about the WTA, but the standard ATA has some bracketry (??spelling, existence ) that is riveted on, and over time if that has come loose or damaged by mechanics will have a sympathetic vibration at certain RPM's
either that or your bottom end is shot, in which case you might as well give me your CS motor, (with goodies) ill take your problem off your hands j/k