[Elecraft] KPA500 Hum when off?

[Jim Miller]

I leave my KPA500 back panel switch in the ON position most of the time and
use the front panel switch to bring it to life.

However in even when in the "Front panel off/Back panel on" state the
KPA500 will occasionally hum for a few seconds.

Is this normal? Any suggested causes to worry about?

Thanks

jim ab3cv