Power Amp transformer hum - intermittent and increases and decreases in a few seconds

Some {not all} toroidal transformers can buzz when DC is present on the power line.

My Son of Ampzilla 2000 has a faint buzz when my Canon printer turns on, and when my Keurig brews a cup of coffee.

Otherwise it is dead silent.

My Pass Labs XA60.5 never buzzes and it has Toroids.