This is a real shot in the dark...and hopefully is not going to cause you more work for naught.
Maybe the cause of the humming is power harmonics?
There's a PDF file at
http://www.fluke.com/ElectricalPower/elec.asp (go to Application Notes, which is on the left menu, third up from the bottom).
The PDF is entitled "In Tune With Power Harmonics" and on page 4 mentions humming panels.