I'm not a tranny guy, but you mentioned that the tranny shop questioned your ground bring good for your lockup. Do you have your engine, frame and body bonded together with good, clean ground straps on bare metal connections? You can very easily check your ground circuit by running a temporary jumper wire from the isolated side of your lockuup switch direct to the battery, then try your lockup switch. If the noise goes away the shop was right and you have an easy fix.