When was the oil last changed? I am guessing at the service but you never know! If so, any ideas what oil they put in exactly? Which engine is it?
Edit: This could go on and on I guess, but basically stuff like that is snake oil. My guess from what you just said is they've put the wrong grade oil in so the top end is now all tappy / clattery. Should be a 5w30 fully synthetic ideally, VW507.00 spec.