I find my 2010 mac pro 5.1. noisy (I know this is a very subjective matter, but a deep, mid-pitched "propeller plane in the distance" type humming sound is emitted constantly) and am trying to find out where it comes from and what I can do about it. Using SMC control I believe the "exhaust" fan to be the culprit. Or it may be a combination of several fans. At least the hard drives aren't the cause of this as I checked by unmounting them all using Disk utility. So which of the several fans is the "exhaust" fan, and is it hard to replace? Having read several postings on the subject I see mention of replacement fans being difficult to source as they should be analog-controlled while the off-the-shelf PC part fans are PWM (digitally) controlled, but I never figured out if this applied to older Mac Pro models or all of them. Finally, I'm having problems with a fan being "stuck" at a higher speed than before (and yes, I've tried resetting the SMC, but to no avail. Actually all was well when I installed SMC control, but having uninstalled it, then installed Fan control one of the fans started running a lot faster (it sounds like a small fan with its higher pitch -the "PS" fan according to SMC fan control), so I uninstalled it (following the instructions at the author's site above, including resetting SMC) and all seemed well... until installing and running SMC fan control again. It seems like its default settings are higher than the Mac Pro defaults. I believe I saw the fastest fans being at around 800 RPMs according to the iStat pro widget but SMC fan control won't allow the "PCI" and "PS" fans to go bellow 1109. Quitting SMC fan control won't reset things, neither will deleting its preference file.