I've just aquired a brand new Danelectro DA-1 guitar pedal power supply
included with a bargain bundle of the newish well reviewed "Cool Cat" pedals.
Its marketed as a Regulated 'Zero Hum Circuit' 9V AC Adapter
and is the correct model for The UK.

Plugged in and using it for the first time first time and everything works fine,
except for a very noticeable constant trebly kind of hum
I've located as being emitted from within the plastic casing of the power adapter.

No unwanted hum from the the amp & speakers [guess the 'zero hum circuit' must be working then ].
But the black plastic adapter unit is itself an irritating source of noise at home practice volume level.

I've never before noticed this 'problem' with any other adapters
from various music and misc electronic product brands.
So is this a malfunctioning unit,
or a side effect of the 'zero hum circuit' ?