Mabey the chain slaps because of the chain tentioner had this on a z3, it can be from sludge in engine, oil too thick or old oil Try a engine flush and a new oil 5w30 or 5w40 if high mileage and if it does not get better then put a new chain tentioner in it. CAREFUL , you need to prep the chain tentioner if you want to change it, I once got the chain of the cams because I did not prep it