built a new building. has 12 4 ft flourescent lights with 32 watt bulbs.(new small style). three switches. one runs two light at end of shop. second runs middle four and third runs six in main section. first and second switches seem ok flip third on by itself an hear humming at switch. gets louder with 2nd switch on with it. the switches have been trouble shot and r not the prob. if they are bypassed the hum is still there. any ideas would be greatly appreciated

As you said it is not the switch.

It must be from the one bank of lights and it is telegraphing through the framing.


Disconnect them one by one to see which fixture that it is.

Then make sure that ballast is tightly mounted to the fixture and the fixture solidly mounted. Or replace that fixture.

eliminated each light one at a time and noise got softer with each light. lights are riveted to metal ceiling. if it was mounting problem would the noise come from light not switch?