Another thought. .. is it slipping or is the derailleur arm chattering back and forth. If it's then the jockey wheels need replacement.
|Chain slipping with new chain, cassette and jockey wheels||Bicycle Mechanics & Repairs||14|
|Chain slipping off bottom jockey wheel||Bicycle Mechanics & Repairs||1|
|A||Chain slips on 21 speed Shimano after replacing chain, didn't slip w old chain..?||Bicycle Mechanics & Repairs||4|
|C||Chain slipping problem||Bicycle Mechanics & Repairs||3|