driveshaft

Not the shaft itself, but the center bearing. The driveshaft is a two-piece affair, and the center section is held up with a bearing. You could be hearing wheel bearings as well ...

"J Strickland" wrote

From his description, it's not clear if the car is moving (rear wheels are turning) when the rumble occurs.

If the rumble/vibration occurs *when the car is standing still* in neutral, either at idle or 2500rpm, it's the transmission input shaft bearings.

Floyd

The driveshaft (propeller shaft?) doesn't go round when in neutral unless you select this on the move.

Or layshaft bearings.

However, gear chatter on an unloaded box isn't unusual. Some makers set the idle speed higher than needed to avoid this.

I was thinking the same thing.

I dialed in on the center bearing thinking that if it was free wheeling, then it might be noisey. It's also the cheaper repair ...

Redline MT-90 oil is quite good for reducing whirr/whine/rattle. Of course the OP's gearbox could be half full of pee for all we know so he needs to check the level (at least) and preferably change the oil unless he's sure it's the correct grade and not too old. Until those things are taken care of he might be causing damage by driving.