My Macbook had a wierd nosie coming from the fan. Its sounded like a ticking noise and got more frequent as the fan speed increased. Its only a week old, but never notice it before, properly because I use it infront of the TV. However, I have use a can of compressed air and blasted the rear ven, which seems to solved the problem. I have loaded the machine upto 4500 rpm and still no ticking noise. Might be worth trying if anyone else is having similar problems.