I have a nice 32gb HP workstation, but it's unusable because the fans are too loud. So I decided to do something about it. I bought three things from eBay..
Computer cooling fans can be loud when they run fast. There's two ways to slow down a fan. One is to use a lower voltage than the 12 volts they're designed for, the other way is PWM, pulse width modulation. PWM means that you switch the fan on and off rapidly, umpteen times per second. Less power, means it runs slower.
My first purchase (£2) is a little board that takes 12 volt input, and has three header blocks for fans, one for a thermometer and one for a buzzer. You set the desired temperature ranges with the dip switches.
Next, I bought a few PWM controllers each for a single fan, £2.25 each.
And third, I bought five 4-pin extension cables, to make life easier.
I installed the board and connected up the two external fans, and the fan that blew down on the memory. This immediately made the box a lot quieter. But still a bit noisy, so I used one of the single-fan controllers to slow down the fan cooling the memory, and that was good.
So I put the board inside a plastic box, to make sure it wouldn't cause a short circuit, and I'm happy with the result.