My u-joint just failed.
Started as a quite whir, while driving two up at 80 - 85 mph but it only whirred immediately after backing off the throttle. I could play with the throttle and recreate it, sounded like purring valves. It was a very quite and cool evening and the wind was at my back.
Overnight, it immediately progressed to a clunking (metal to metal rub) on decel at about 2500 - 3500 rpm and sounded like it was coming from the timing belt area. It didn't initially sound like it was originating from the back or the middle of the machine.
Within 20 miles, it was occurring almost immediately after starting forward movement but only when torque wasn't being applied. Come to find out the torque was binding the arms together quieting the clunking, on decel they separated and clunked around.
This was my experience, your experience and results may vary ... but do let us know what you find and how your hunt goes.