Is the hum mechanical?
If so it can be related to DC in your AC. Try turning off every other breaker in the home to try to locate it.
Hegel hum?
Spent a week or two enjoying my new H400 and noticed a faint hum between the time I turn it on and before I play music. Also there when I pause after playing a while. Note that the hum is only apparent when I bring my ear a foot away from the amp. I’m quite pleased otherwise but am wondering if this is normal?