The toroidal in my F5 clone hums surprisingly loudly. Virtually no noise at all through the speakers, but you can hear that toroidal humming away from 2 or 3 feet away. It almost makes a grunting sound when I power it up.Toroids saturate easily if there is DC on the AC line. A common cause of this might be a heater running at half power- it is only drawing power on one half of the AC waveform. This causes half of the waveform to have a lower amplitude- and thus can be regarded as DC on the AC line.
Its easy to block with a simple device, and may silence the transformer. The amp might also sound better too, as the transformer might then deliver a cleaner waveform to the power rectifiers.
The grunt at turn-on is common with many transformers, toroidal or EI core.