just the last couple weeks i have noticed a driveline vibration and i cant figure out what it might be. It occurs only in 3rd gear with the converter unlocked from about 3000 to 6000 rpm. Now, it doesnt show up at this rpm range in any other gear, so its probaly not the engine, flywheel or converter. Also, in second or fourth gears at the same speed, the vibration is not there. So its not the driveshaft, rear end or tires.

I recently installed a poly trans mount that definitely made it more noticable but it was still detectable with the rubber mount.
I seem to have eliminated all the rotating components so im stumped now.

could it be some type of cavitation between the stator and impeller? its a CPT race trans and street edge 3000 converter, 3:42 richmonds, and stock drive shaft. if anyone has any ideas lemme know.