Here is my retro PC that is totally silent and has no moving details.

It's Siemens Nixdorf PCD-4Lsx.

Specs:

Socket 1 486 DX2-66@50
4 Mb SIMM-30
Cirrus Logic CL-GD5428
512 Mb CF
GOTEK floppy emulator

Pros:

- silent (even the PSU is passively cooled)
- small
- light
- easy to service

Cons:

- only one ISA slot (however, it's full length)
- cannot install high SIMM modules due to small clearance towards the ISA card
- CMOS battery is dead
- no CD-ROM
- no reset button

Plans:

- change the CMOS battery