Keeping a small home lab quiet
The cheapest noise reduction is fewer machines. Before buying quieter fans I retired two boxes whose only job was being on. Everything they did fits on one small router and one low-power mini PC.
Second: let things sleep. Spinning disks that are only read once a week do not need to spin all week. hdparm -S with a sensible timeout cut both noise and the electricity bill.
Third: move the loud thing. A closet with a gap under the door is a surprisingly good server room for one machine. Measure the temperature for a few days before you trust it.
What didn't help: fan curves. Tuning them bought a few decibels at idle and nothing under load, which is when I notice the noise anyway.