that sounds like a bad catalyst. very common usually the noise happenes when you are at idle and rev it up or driving over bumps.
+1 He should try revving the engine whilst in park and see if it makes this rattle noise.
"I am enclosing two tickets to the first night of my new play; bring a friend, if you have one
- George Bernard Shaw to Winston Churchill.
"Cannot possibly attend first night, will attend second ... if there is one
- Winston Churchill, in response.