I built a system around an A4-SFX case to have a portable, reasonably fast and upgradable system. These have the size of an external GPU but can house the entire PC (https://www.dan-cases.com)
Hardware has to be specifically chosen to keep noise under control and to fit. E.g. the GPU can only be 2 slots (2.5 slots for the new version) wide, and only smaller CPU coolers (or quite small AIOs) fit.
Running a 65W Ryzen 1700 CPU and a reference design, slightly undervolted GTX 1080 TI.
The system is unhearable in idle and not disturbing under load.