This is not uncommon - I am working on the same issue - there are CPU and Power Supplies that you can buy that are made to run quieter than the stocks ones that come with many systems - a bit pricier but worth it - especially if you do computer based recording. Unless you don't have any bearings going out in your fans then upgrade too better fans and power supplies - God bless
Your computer case is something a kin to a kettle drum. You attach that vibrating hunk of plastic to it with fat screws and it vibrates the whole thing. You can go several routes. The first is what I did at first. The second is what I did as a follow up.
!> Install a good fan controler. Mine is a Cooler Master Aerogate II. 4 fan controls and 4 temp sensors. $39.00USD.
2> Install silcone fan and psu gaskets. A kit of 2 fan and 1 psu gasket comes with screws and extra washers. $17.00USD
It works great on my el cheapo case. When I'm on the system I dial up the fan speen to match the job and when done turn then down. Can't wait to test that set up on my clear case.
Some stock fans are quiet but are loud in small spacers like when i had some AVC fan that came with my PC it was on my Heatsink and make a huge whizzzzing noise so i got a coolmaster fan and stuck it on, also try and get fans somewhere about 20dB
Just A Guy With A Voice In His Head...