Making a shelf server quiet enough to forget
The server sits between a row of paperbacks and a houseplant. That was a decision made for the sake of domestic peace, not thermals, and for the first month the fan let everyone know about it.
Measure first
I logged CPU package temperature, both disk temperatures and fan RPM once a minute for a week before touching anything. The picture was clear: the machine idled at 41 °C and the stock fan curve reacted to every short spike as though it were an emergency.
What helped, in order
Hysteresis. By far the biggest win. The fan no longer changes speed unless the temperature has moved by four degrees and stayed there for twenty seconds.
A larger, slower fan. Swapping the 80 mm fan for a 120 mm one on a printed adapter moved the same air at roughly half the speed.
Spinning disks down — carefully. The bulk disk now sleeps after 30 minutes. I checked the load cycle count for a fortnight afterwards to be sure nothing was waking it every few minutes; an indexing job was, and moving its database to the SSD fixed that.
Rubber feet. Embarrassingly effective. The shelf was acting as a soundboard.
Result
Idle noise went from clearly audible across the room to something I have to put my ear next to. Disk temperatures peaked at 39 °C during the August heat, which is comfortably inside what the manufacturer asks for.