As rand_79 says, when you slow them down or replace them with something that has less cfm you will be loosing some cooling. Do you have them plugged into the cpu fan header? If not try that. Also, with that said - Do you have the motherboard controlling the fan(s) speed? If you plug it into the cpu fan header and your motherboard is controlling your fan speed they shouldn't speed up and make all that noise untill your processor starts to heat up. Lots of people complaining about the really loud 19dBa of noise coming from them, don't believe it's as advertised. If you replace them with fans that have the same cfm's air flow with a lower dBa you should make out better. The noctura's you picked out will fit but don't turn as fast so probably won't cool as much. also, pull the side off your case and see if they continue to spin at high speed and noisy. It might be it needs better air flow through your case and that's what's causing them to spin fast and be noisy all the time. Other option is to buy a fan controller but when you need the cooling and have to spin them up you will have the same noisy problem, just not as ofter.