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.

So looking in the IPMI interface, I noticed the Low 'Non-Recoverable' and 'Critical' levels for fan RPM were set kinda high—at least as far as Noctua fans are concerned (ignore the fact that FAN1 is reading as off—I had switched fan plugs earlier when I was moving around a system fan...). Since I bought slightly oversized fans so they could run at lower rpms (and thus quieter), the server-oriented motherboard didn't know what to make of them.

Typically one installs whiny siren-like server fans that spin at 5000+ rpm, and don't worry much about noise. But I do.

So to get the system to accept that it's normal for a fan to go below 500 rpm, I installed ipmitool (sudo apt install -y ipmitool on Debian), and ran:

ipmitool -I lan -U ADMIN -H [IP] sensor thresh FAN1 lower 150 250 300

Replace ADMIN with your IPMI username, and [IP] with your IPMI IP address.

This sets the NR rpm level to 150, LC to 250, and lower 'non-critical' to 300. Much more suitable for Noctua fans that commonly run under 1k rpm. And IPMI was happy again: