As the Fan blades normally are very close to a panel/pc case, you get a lot of turbulence.
Noctua researched this case and developed a spacer to reduce the loudness on average by 2dB(A) & up to 3,5dB(A) and increasing the average delivery capacity of 4.4m²/h - up to 8,3m/h (*).
I used that design and made a simple 5mm Spacer for the most used PC Fan sizes.
As all fans & PC housings have the same dimensions, the printed spacers can be used universally on all brands.

You can also print it in TPU as this would have another additional improvement of being vibration-damping.

I wouldn't have thought that such a simple solution could bring the sound down so effectively. I can only recommend giving it a try. Design is super simple and everybody could probably do it, but not everyone knows the "trick" and so it may reach more people by uploading it here and not having to look for dimensions or measure it.

(*) [https://noctua.at/en/noctua-introduces-na-is1-inlet-spacers-for-suction-applications-and-na-savg2-gasket-set](https://noctua.at/en/noctua-introduces-na-is1-inlet-spacers-for-suction-applications-and-na-savg2-gasket-set)