Building a homelab that lives in a cupboard
Noise is not a spec you check at the end. It is decided by four choices you make at the start, and only one of them is fixable afterwards.
The rule: silence comes from small fans spinning slowly or no fans at all. Any design that needs air forced through a narrow chassis is loud by physics, and no purchase undoes that.
The four decisions, in order of impact
1. Chassis height. This is the whole ballgame. 1U forces 40mm fans at high rpm. 2U allows 80mm. A tower allows 120mm+, which can move the same air at a whisper. If silence matters, the decision is made here and everything else is detail.
2. CPU choice. A chip with a low TDP does not need much cooling in the first place. An N100-class mini PC (Amazon search, opens in a new tab) is passively or near-passively cooled and is the easiest silent build there is. This is why most quiet homelabs are mini PCs and not servers.
3. Drives. Spinning drives click and hum, and several of them together produce a resonance you will hear through a cupboard door. SSDs are silent. If your cupboard is next to a bedroom, an all-flash build is worth the capacity penalty; otherwise put the spinners in the box furthest from people.
4. Fans. The only genuinely retrofittable one. Better fans (Amazon search, opens in a new tab) in a tower or 2U case can take a build from irritating to unnoticeable. In 1U they change nothing.
Cupboards have a second problem
A closed cupboard is a heat trap. A silent build that cooks itself will thermally throttle, and then it is slow and the fans ramp up, which is the worst of both. Leave the door ajar, drill a vent, or accept a passive temperature rise of a good few degrees and pick components with headroom.
Also: put a UPS (Amazon search, opens in a new tab) outside the cupboard if you can. Its own fan is usually silent, but its alarm is not, and you want to hear that one.
The picks
Silent build that runs everything: N100-class mini PC, all-flash, no discrete GPU. Genuinely inaudible in the same room.
Silent-ish with real storage: mini PC for compute plus a two-bay NAS (Amazon search, opens in a new tab) sited away from people. Splitting the boxes lets you put the noisy half somewhere else, which is the cheapest noise fix available.
If you already own something loud: better fans if it is 2U or a tower, a different room if it is 1U. There is no third option.
Do not buy a used rack server "just to try it" if it will live near people. It is a common regret, and the steady supply of them on the second-hand market is the clue.