How to deal with noisy cpu fans?

Hi im using a grow box and im growing indoors for the first time. Now i have 3 cpu fans 2 out 1 in for smell control. The fan taking air in to the box is making a humming noise which is low but if you hear it , it is noticable and i have people over alot so i cant really let them hear it. Does anyone have any suggestions on how to make the fan at least a bit quiter? if not then should i just get a new fan? o the fan im using is old but the other 2 are just as old. thanks.

put a rubber frame between the wall and the fan to reduce vibration

Ya it depends if its noise from the fan or vibration, i would go with d1z13's idea of rubber around the frame or even weather stripping or foam. If the fan is noisy itself you can try using a 9v adapter instead of 12, this will decrease the power about 25%

have the same amount of cfm supplying as you have exhausting. pressure differences make the fan blades vibrate which makes them noisey.
also rubber between the fan housing and whatever you have it bolted to will help alot.
i wouldnt try the 9v trick, it will burn out your fans!

k i tried the foam idea since i had it and it seemed to fix it completely thanks.

umm the noise is still noticabele from like 20 feet away if i put like objects around the closed area would it break up noise waves a decrease the sound a bit? not expecting a big decrease just enough so i can have people about 5 feet away and not really pay enough atention to the noise.

put a freezer or pc next to it .... and im using adapter with controlable voltages from like 3v-12v pc fan is starting to spin from like 3volts (at least some of them) and its really really quiet!!! only thing is that cfm decreases too you could put in a thermometer in box ... lower voltages to fans and see how the heat rises and put on such speed so the temperature wont rise too high!!! i used any kinds of volts and my fans still work after 3 months like at the beginning

put the fan somewhere else and attach it to box with ducting.

If u are handy you could get a plug for any electronic equipment, the kind with a big heavy converter the plugs into the wall, everyone has these laying around in their home. you can rewire different plugs with certain mA ratings till you find a speed that isn't so loud, but you will suffer air flow with a smaller voltage, I use plugs with a 730 mA rating and they are barley noticable with my computer fans so I would try a 300-500 mA plug.