You didn't mention if you are using 110 or 220V for your input.

Most server PSUs are designed to be ran at 220V, although they can operate at 110V with a reduced power rating.

This may be one of the reasons they are running so loud, as although they can operate at the lower input voltage, this comes at the cost of twice the current (amperage) and in some designs can put out more heat then the same wattage with a higher input voltage due to the internal design of the circuitry. Many HP server PSU will have two ratings, something like 1200 watts at 220-240V input, but only 900 watts with a 110V input for these reasons. Better check it out and get back to us.

BTW, I power several rigs with HP server PSU's, but run them all at 240V (US) input and they may spin up a bit at first, but they quickly quiet down and run no louder than a normal consumer grade PSU after a few minutes.