Autobox rumble
 

I finally got my 16v IACV issue working, so it's on to the next problem!
In between ~600rpm and ~950rpm, my car rumbles quite badly. I'm guessing this is the gearbox, as it changes with revs. Any ideas? It's an auto by the way.
It goes away as soon as it's out of that rev range, and is fine when moving.

Ck your accesory drive belt(s). They can pick up low speed harmonics and make a rumble sound. Check for proper tension on them.
Open the hood with the car running and CAREFULLY rev the engine to the "sound" and see if the belts vibrate.

The drive belt is new, and I've tensioned and retensioned it many a time since this has been happening. It's been doing this for a while, but I assumed it was down to my bad idle. :(