Strange noise

Hi, I've got a strange noise which appears to be coming from the rear of the bike, when out of the saddle going up hill, in whatever gear, there is sometimes what sounds like a grinding noise?? any ideas what it could be? it started about 20 miles or so ago.
Quick Edit :- Some info on the bike. Its a Ridley Orion 2010. With Shimano 105 and Easton EA70 wheels if thats of any use.
Quick Edit :- Some info on the bike. Its a Ridley Orion 2010. With Shimano 105 and Easton EA70 wheels if thats of any use.
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aka need better grease thou!