One more possibility: Noise on the A.C. power leg. In P.A. applications we deal with this quite often. As others have discussed, it is caused by another appliance on the power leg that puts out a secondary frequency that ends up being superimposed upon the 60hz frequency of the A.C. mains from the wall. Lighting dimmers are notorious for this because they chop normal power into variable duty cycles to reduce the light output of a bulb. In doing so they squirt a stream of square waves into the power line. There can be other sources of this noise but the results can sound just like induced hum.
The way to get that off your line is to use a power conditioner. Use of a power conditioner is also the cheapest way to diagnose it.
Bob
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