it sounds like a loose chain slapping on the swing arm to me. I know you said he checked the tension, but what is it at at the moment? chains may have tight spots, so it's best to check the tension, rotate the wheel and check it again, rotate and check, rotate and check. that way you will find the high points and low points of the chain. ideally you should set the tension at it's tightest point to stop the chain snapping (which is rare, so don't worry about this) or wearing faster. with the tension set, if at it's loosest point, the chain hits the bottom of the swing arm, you may need a new chain.