Well, whatever I've been doing to my MTB recently, it seems to be rebelling.
When I'm cycling fairly fast there seems to be a bizarre buzzing noise coming from the middle of the bike. It seems to buzz with the tyres - it doesn't do it at a low speed (less than about 12mph) but once tyre noise starts to become noticeable it seems to speak up.
Upon further inspection (hitting the saddle with the palm of my hand to replicate bumps in the road/tyres) it seems to be coming from something at the back. I'm beginning to wonder if it's something to do with the wheel bearings - does anyone have any opinion on what it could be?