I don't normally chime in with "me, too" posts, but in this case perhaps reiteration is justified. You give too little information in your post, you'll get suggestions that are, justifiably, all over the place. It could be the chain ticking the spokes if the limit screw is not adjusted properly. It could be you have to feather the rear derailleur. It could be worn chain/cassette. It could be that it's not the gears at all - pedals? bottom bracket? bars? saddle? Etc., etc.