your not overloading it, but you are bringing its load up pretty high for it to scream with that little fan it has. if you got a slightly more wattage server psu, you wouldn't have that problem, say like a 1400w or 1500w if you want to keep it at just using 2 psu's per rig
Would it be an idea to get 2 more of the same server PSU's and have the unit running from 4 units?? I really need to kill the noise