Car rumbling... Any suggestions?

[ChiChoAndy]

OK, just got back from doing some shopping in town, and on the way back, the car made a weird rumbling noise, at 60 kph+. if you depress the clutch, it goes. If you let your foot off the gas, it goes. There is no vibration coming up the gear stick, steering wheel, or the 4-lo selector. Can feel it most through the floor. It's fine at lo speeds.

I reckon it isn't a bearing, as it doesn't change 'speed' as you are speeding up, or down,and doesn't change going round a corner. The engine revs fine, and doesn't seem affected when out of gear. Gear change is fine, as well as the brakes.

Any ideas? I've an inkling it could be clutch related. Clutch is working fine, with no slippage, etc, and I can't think it would be anything else.

The floor is open!