If the subwoofer only hums strangely from the inside instead of delivering fat bass when it is put into operation, you would naturally like to get rid of the humming immediately. That something is wrong with the subwoofer because of the humming is obvious, but can be ruled out in almost all cases. The subwoofer has probably just "caught" a hum loop.
Humming loops or other externally acting factors are often easily remedied by sound disturbances. We explain what you can do about it - here: Ground loop