Okay, a further update. While the out of balance start-up fan noise temporarily stopped for awhile. Now she is back, and it is not the APC power supply nor is it the dual fan assembly in the front. It is, also, not dust, as I used compressed air to keep dust out of my Mac Pro. So I stuck a part of a small six inch piece of heat-shrink through the back of the aluminum cage into the rear exhaust fan during boot and the noise has gone away again. I will be replacing the rear exhaust fan in the near future and will report back. BTW don't try to replace any Mac fan without replacement mounting grommets, which you can get from Tiger Direct or Amazon ( "Silenx IXAFM20 Silicone Dampening Fan Mounts - 20" ).