How’s the chain tension, loaded versus unloaded? Age of chain? Stretch, etc… you can get a ridiculous amount of noise/clicking/vibration from an old chain that’s nearly binding on deceleration and almost getting pinched in between the crankcase and the chain guide and /or around the front sprocket, or even slapping against some part of the frame on it’s way toward the front sprocket.
Easiest way to ‘check’- put it up on a stand (well secured), start it up, put it in gear, give it some gas and let off to see what the chain does under decel.
When you chain/sprockets are rather worn, you have enough slack in the drivetrain to actually bunch up and bind in some of the spaces where the chain normally just slides through…