What kind of case do you have?
Are these fans the ones that came with your case, or did you buy them?
Describe the fan sound for me.
I have 5 fans + the ones in my PSU.
The one that sits on my proc produces a midrange frequency and depending on what I increase the voltage to it gets louder on that same frequency.
Then the PSU fans for me produce a deeper hum but it is the loudest in my case.
Finally my other case fans produce in between high pitch to medium (due to the vibration).
One thing some people suggest is to use dynomat (or some other sound dampener), however, I feel it makes your case very hot and unless you aren't doing much overclocking or don't really bother with system temperature (as long as the computer is stable) then I suggest that. Otherwise, I suggest you power up w/ different fans and figure out which one creates the noise, then eliminate it. For me it's the PSU so in the future I plan on buying this ...
Once I see some reviews for it of course