I've been building a Mini ITX 'quiet-ish' server using a Supermicro motherboard and some Noctua fans.
I noticed sometimes the system would start 'revving' the fans up to max power. Then after a few seconds they would get quiet again. The CPU temps and other temps on the system were stable and not worrying, but popping off the server's cover, I noticed LED8 on the motherboard would blink red every time the fans would ramp up:
That LED indicates a 'fan failure' when blinking.