The fan on my xbox is extremely loud/running very high all the time, but my other xbox is almost silent in comparison. Any idea why?

I have two xboxes, we'll call them A and B. They're both old white ones with no hdmi and the same motherboard. I tried swapping out the fan from the B (quiet one) with the fan on A (the loud one), but its still loud. I'd like to try and fix it if possible as A dumps out much more heat than B.

Any help or suggestions would be greatly appreciated. =]

edit: Forgot to mention that all the games I play are installed and it does it constantly, even if its not been on for long/I'm not playing anything.