Had a case of that the day I was out around North Cornwall. All good for 80 miles or so but in the last 10 miles I could hear this sort of whirring from the back of the bike, started quiet but became more intrusive. Anyway I stopped and checked out the rear end, nothing amiss, no play in the bearings or noise when spinning the wheel, suspension all where it should be, brakes as they should be, tyre pressures right on, tyres all good, chain tension good. Couldn't find anything obviously amiss. Rode home and it was still there.
Anyway that evening I thought about it and worked out that I hadn't lubed the chain for the last 4 tankfulls. So about 650 miles. Next morning I cleaned the bike, cleaned and lubed the chain, went for a spin and the noise was gone.
Two things: I was relieved that the noise was gone but pissed off that I had missed doing such obvious maintenance. I am normally very good at that sort of thing and lube the chain every second tankful but obviously had a brain fart this time.