Recently my chain has started making strange noises, it sounds as if i is grinding or not sitting on the drive sprocket correctly, but its lubed well and the tention is correct. A mate even noticed the sound when riding behind me. Ive had it serviced recently and they didnt say anything (mind you they didnt even bother to talk about the bike to me at all, and no im not going back). I cant think what it would be. Any ideas, THANKS.