I think I know what the OP is talking about.
My unibody MBP was dead silent for the first week and then last night (day 7) the fan started to click. I noticed it when I was doing CPU intensive tasks. It sounded like a diesel truck, and the noise would kick in at around 4000 rpm. When the fan started to accelerate, it sounded like a a jet taking off. Now I hear a faint click click click even at 2000 rpm.
I will be taking it into the apple store this coming friday, earliest I can go. It'd be 13 days since I bought. I hope they let me exchange it.
Do you guys think a genius will have a way to accelerate the fans up to 5000 rpm?