1. Buy a OEM replacement fan (direct from suppliers in China or locally) and throw it in yourself with the help of iFixit
See if the stock fan is loose (Fan itself or how it is mounted) you don't touch the mobo aside from the 4 mounting screws to hold the fan.
Still making the rattling buy a OEM fan direct from suppliers and throw it in with new/better TIM and see if its still faulty and making those odd noises. I never had rattling noises at all yet just the normal high pitched fan when I'm rendering so thats the only two things I can think of.
I average around 45-55C doing my normal workloads (Adobe, Chrome, etc) or 65C (if external monitor) and tops at 90 - 100C for rendering with the stock fan. Been around this area for the past 12 months on top of the time it been with the first owner still running like it was new, temperatures have been consistant with what others experience and is healthy.
2. Go though Apple store/authorized repair centers to avoid you causing anything if your paranoid about the fan.
Although expect to pay for parts, labour and possibly a inspection fee.Edited by ivr56 - 7/3/13 at 5:55pm