Fan speed

The fans in your PC are loud because either:

a) their design is inefficient, or
b) something is causing them to spin faster than they should.

If it's a) there is very little you can do short of replacing them with low noise fans. I once built a system with horrendously noisy fans. Nothing I did could quiet them down, so I ended up having to replace them with new ones with a lower noise rating and that solved my issue.

If it's b) then you need to look into the root cause for the excessive fan noise. A good place to start is the system BIOS. Check for settings that can affect case or CPU fan speed, such as thermal management. If the settings are too aggressive they may be causing the fans to speed up more than necessary. If there is a fan speed override that was left on turn it back off. Make sure when you are testing the sounds, you will miss the changes that you will notice after changing the settings so make sure don't be lazy. Since it sounds like you've already covered those bases, verify that you have good airflow inside your case. Intake fans should go at the bottom; exhaust fans at the top. Of course, you also need to make sure they're blowing air in the right direction.
At the end of the day you may just need to get silent fans if none of that helps.
My suggestion is to always start by cleaning the inside of your PC, then playing around with the fan speed presets in the BIOS.
Hope you found it helpful and it truly helps.
Regards