PC case fan loud?

Is this generally the case or just a few special cases?

This is always the case

1. has an object close to or behind the fan and pulls the air against it,
2. the air moved through narrow slits instead of the entire surface of the fan frame
3. for any inexplicable reason PWM fan installed and not properly regulated
4. too fast fan installed, would not go over 800 rpm, the slower the better

You’ve got it with cheap and expensive cases. The absolute majority of enclosures, just has a bad design.

Has a well-known one who has built a PC a short time ago told me about “cheap” cases,

Your friend has no idea. In cheap cases, slow 3 pin fans are usually installed with 500-800 rpm, which are not loud.

However, if you can still very cheaply install quiet fans and is below the price of most other housings:

https://geizhals.de/nanoxia-special-n-v-120-800-nxnv120-l-a1563759.html

And finger off the Deepcool case.

Good ATX housings are e.g.:

https://geizhals.de/deepcool-matrexxx-55-mesh-black-dp-atx-matrex55-mesh-a23013.html

https://geizhals.de/aerocool-quantum-mesh-v1-black-quantum-mesh-g-bk-v1-a2408240.html

μATX housing:

https://geizhals.de/lc-power-gaming-712mb-polynom-x-lc-712mb-on-a2799380.html

https://geizhals.de/aerocool-trinity-mini-accs-pv32033-11-trinity-mini-g-bk-v2-a2375595.html