either I'm too lazy to let the video load all the way, or the damn thing ain't working. at any rate, what was it at the mechanic's shop for? That might help us grease monkeys figure out what's wrong with it.
Typically, a slapping noise from the back of the motor is usually a timing chain tensioner. If it's low on power, there's a possibility that somehow the tensioner went back and the cam timing chain jumped a tooth...or they pulled it off for some reason and screwed up putting it back together (i did that once..:doh).
Either way, once we find out what it was at the mechanic's for, it'll be easier for us to diagnose.