canadians14sr20det

I just finished swapping a z33 (cd000, yeah I know...) box into my s14 with sr20det. I'm getting a crazy resonance vibration at 4000rpm in any gear, and the sound isn't localized. It sounds like its in the trunk/shell of the car. The noise does not happen when free revving the car, only under load/decel.

The z33 swap was done via the cut and shut method, with the input shaft lathed down to match the sr20 transmissions. The two halves of the transmission were located via a jig setup to run through the main journals of a spare engine. Car shifts and drives fine, I doubt it's the cut and shut itself.

I've smashed out the tunnel to ensure there is at minimum an inch of clearance everywhere around the transmission. Doesn't seem to help. Can't hear the transmission clunking into the tunnel either. Anybody know how much a transmission moves under load? The motor and transmission mounts are stock 240sx. The transmission crossmember was made by cutting the stock member and tig'ing it to angle iron.

Normally I can track these things down and fix them, but this is perplexing me. Anybody got any ideas?