Originally Posted by
cny-bikeman
For future reference a once per pedal revolution noise cannot be anything at the rear of the drive train (chain, derailleur, cassette) nor the chain.
According to Sheldon Brown, they can:
http://www.sheldonbrown.com/creaks.html#bottom
Chain-related noises, occurring once per chain revolution
(every 3-4 pedal revolutions, depending on gearing):
This indicates that there is something wrong with your chain. You may be able to fix it, or you may need a new chain.
Stiff Link?
If you have a regular, repeating skip or hitch every 3 or 4 turns of the pedals, you may have a stiff chain link. This is commonly the link where the chain was joined when it was installed. When the chain tool presses the pin through the chain, the head of the pin tends to pull the uppermost chain plate along with it, so that the two outer plates are squeezing together against the inner plates.
The easiest way to fix this is to bend the problem area of the chain into a "Z" shape, with the bad joint on the diagonal part, then flex the chain back and forth. This will slightly spread the tight plates, and free up the link.
Bent Link?
If your chain has a link that has been bent in a chain-jamming incident, it can cause similar symptoms. An emergency repair can sometimes be effected by bending the link straight, with a chain tool to one side of the bent link and an adjustable wrench to the other, but even if this succeeds, the bent link (or the whole chain) should be replaced.
The easiest way to spot stiff/damaged links is to shift the bike into the small/small gear (the gear you should never actually ride in.) This gear has the chain at its slackest, and flexes it farther than any other gear, as it goes around the small rear sprocket and the derailer pulleys. Slowly backpedal while watching the chain as it feeds through the rear derailer, and you will usually be able to see the bad link jump.
Mixing New and Old Chain?
If new links are added to a worn chain, there will be a light clunk when the transitions between the shorter (new) and longer (old) links pass over a sprocket. This can occur even when only installing a new link pin or master link. When a longer chain is needed, use a new chain."