oh and fyi on the fans.. iirc all mac fans idle at 1000 rpm, which should be silent for all of them. the smc speeds them up in response to a rise in temperature somewhere in the zone the fan cools. macs have from 3 to around 16 temp sensors scattered around, and 1-3 fans, many more for xserve and mac pro. Fans only spin a fast as they need to, they are individually controlled, by RPM. I think 5000 is the max rpm for all the fans. (may have seen a 7500?)
Fans have tachs, and the smc knows how fast the fan is going. If a fan fails to perform properly, spins too slow, fails to spin fast enough, fails to get up to speed fast enough, the rpm sensor fails, or if any temp sensor fails or gets otherwise out of spec, the smc will crank up fan speeds. Possibly all fans, possibly just one or two, as a safety measure. Then you will hear them.
It may only spin up one fan to max. If a temp sensor for the optical drive fails, in an imac, the fan that cools the ODD and gpu will go to max rpm, but the othe fans will continue to work normally.
It still surprises me that I never see a windows pc, desktop or laptop, that has controlled fans. They all just spin them at max all the time, and you always hear the fans. Desktop that's some kind of law. I think I may have heard a quiet pc laptop or two though?
