From his description, it's not clear if the car is moving (rear wheels are
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.
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.
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