I decided to replace my chain and sprockets, so last night I took them off and put the new ones on(DID chain, Vortex sprockets). When I was done I spun the rear tire and there is a knocking sound coming from somewhere. It seems to be a bit sporadic. I tried continually spinning the tire to see if it was one particular spot on the chain that it does it in, but it doesn't seem to be that way. Any ideas on what might be making this sounds?