Greetings and welcome to SPCR,
It has been a while since most of us have even dealt with a 25mm thick 80mm fan, but the same principles apply. The bearings and motor, and the frame material and geometry, and blade shape and RPM range - are the main factors.
For example, this fan has ball bearings and opaque plastic (both good things), but it has 2500RPM and the trailing edges of the fan blades are nearly parallel with the support struts (which creates noise):
https://www.directron.com/ec8015m12ca.html![Image](https://www.directron.com/media/catalog/product/cache/1/thumbnail/600x/17f82f742ffe127f42dca9de82fb58b1/e/c/ec8015m12ca_MAIN_1.jpg)
It also has just 5 blades, which means it is designed to work at higher RPM's - fewer blades for higher RPM.
This fan has many more blades:
https://www.quietpc.com/zm-op1![Image](https://www.quietpc.com/images/products/zm-op1-1.jpg)
... but they are relatively straight (and are not swept much) and they are steep angle of attack, so they will move more air at a given RPM, but probably be louder than if the blades are swept a lot and had a less aggressive angle of attack.
This fan is likely quieter, though it has a sleeve bearing, and may not last as long:
https://www.newegg.com/Product/Product. ... 6835103081![Image](https://images10.newegg.com/ProductImageCompressAll1280/35-103-081-03.jpg?w=660&h=500)
The blades are swept forward, with a lower angle of attack, and they are nearly perpendicular to the struts.