Car: 97 DX Hatch, 99 EX trans. Stock aside from ES motor mounts and shift stablizers.

Just swapped a transmission I picked up into my daily. On deceleration in just about any gear, there's a noticable whine. In first and second gear, it's pretty damn loud, almost as loud as a bad ISB in second, louder in first. And again, that's only when I let off the gas. Give it some gas or press the clutch, and it's silent. Under power it's fine. third gear is a little noisey, but no grinds, slips, anything except the noise on decel. Shifts like butter, and feels great under acceleration.

I'm not super worried, as I can easily replace the ISB in the DX tranny and swap it back in, but I'd just rather not go through the hassle.

It's not the throwout bearing, it only makes the noise when the clutch is engaged.
It's not the input shaft bearing, as it's as quiet when under power as it is when in neutral or when the clutch is disengaged.

Thoughts? Suggestions?

 

Could be a diff bearing. When you decel under load, the entire differential carrier pushes to one side of the trans case. If it pushes against a bad bearing, it'll whine.

 

That's really the only thing I could think of. Another suggestion from a friend who has LARGE amounts of experience shredding transmissions (in subaru's, I think he's on WRX #5? lol) was the possibility of a poly mount somehow forcing a misalignment of some sort, and somehow causeing extra NVH to go through the chassis.