Disableing it will cause your video card to overheat, But who cares about an MX400 anyways? Not like you could not get a FX5200 for $49.
But the Cheapest option in this case would be to replace it.
http://www.bfgtech.com/fan.html It takes 5 min or so and its all done.
http://www.computerbrain.com/a...u=BFGGPUFAN&sc=frg
And I saved the best link for last, this one does not need or have a fan, so it will never bother any one!
http://www.newegg.com/app/View...=14-145-095&depa=1
PS try taking the heatsink off of the card and try cleaning it (do not get it wet). Also see if manualy spinning the fan helps to clear it up. Manual Spinning can help get debris out and alows you to possibly find the source
of the noise within the fan itself.
I have one that i will sell you for $11 and free shipping.