Had a similar problem and it turned out to be a bad freehub on the rear wheel. I took the wheel off wrapped a towel around the cassette and turned against it and it would slip ever so slightly. When I took the wheel in to have it looked at the guy thought I was crazy at first, checked the rest of the bike and sent me back out with it. I promptly returned with it and said the problem was still there and he decided to order a replacement freehub since it was warranty work anyways and found that it really was what was causing the sound. It was a nasty "ping" like noise whenever there was a significant amount of pressure put on it. Dont know if this is whats wrong with your bike but its something you could check.