The face-shovel-whacking is a great idea, but I would first have a good talk with him and obligate him to give you at least $500 for his little joy ride.
My guess would be that while he was slip shifting, he missed the spot where the syncro teeth match up and decided that instead of correcting his mistake by pushing in the clutch, he jammed it in anyway thereby snapping, cracking, and popping of a couple of the necessary gear teeth and that is where the rattle is coming from. Just some gigantic pieces of metal rattling inside your trans.
There are 4 perfect points of shift where you can slip (hence slip shift) into the next gear without using the clutch. Nine times out of ten, the only people to do this are named billy bob, smoke ten ciggs at once, and operate a little dinghy called a Peterbilt or a Kenworth on west coast turn arounds. And they deal with about 30 gears, not 5.
So get your money out of that moron or the monkies that raised him, then flatten his nose with a spade.
*drops in his $0.02.*