I had apple fix up my old 17" mac pro for my dad, a couple months later it started making this awful rattle noise. Turns out it was the right fan!
Followed the instructions, was awesome, only thing that was kind of tricky was removing the tape from the old fan, and kind of just rolling it back on to the new one... no so sticky after 5+ years I guess. oh well fan works through.
Wish I had fixed this sooner, or just replaced both left and right fans while I was at it.