Clean and lube the shit out of your chain, I would be willing to bet this is what it is. Put it on a rear stand and rotate the tire and look for tight spots, if you don't find any clean it very well, take it for a ten minute ride come back and put it on a stand then spray it with chain wax while the chain is still warm and then let it cool down. Also make sure the chain is aligned correctly.
How many miles do you have on the bike?