This morning I woke up my HTPC from hibernation and immediately heard this rather loud fan noise. After the PC was fully up, I recycled power. The noise was gone, but when I ran "Hardware Doctor," a utility that came with my ABIT NF7-S motherboard, it showed the PSU fan was off. Thing is, I can't remember whether the PSU fan is usually off when the PC first starts and comes up later (I usually checked the CPU stats only), when it warms up, or whether it is supposed to be on all the time. This PSU is one of these ultra-quiet ones that came with this ATC-613B case .