the universal charger was probably very badly or not at all, filtered...a
proper step down converter powering a radio of any sort should have filter
capacitors inline to filter the AC cycle hum.
I think allot of chordless phone bass units that are plugged in are using
non-filtered converters in their wall warts, that causes hum to go through
the transmitter and you can hear it on the handset while in a call.  Same
would apply to any receiver plugged in, you will hear a hum coming out of it
from somewhere if you plug it into a universal converter that has no
filtration...if you switch to a more expensive filtered supply, hum can
usually go away.
73
Colin, V A6bkx