Electrical interference f

Hi Darro!
D > I forgot to mention that my all-battery radio hums too.
"'Cause it doesn't know the words!" Ooops; sorry: wrong joke! <g>
Seriously, the answers on failed/marginal filtering on the line- operated radios are therefore incorrect as the hum also occurs on your battery radio. The battery is DC (direct current) and in itself would not cause a hum. The line-operated radios derive their power from the wall outlet, which in the U.S. is 60 Hz AC (60 cycles per second alternating current). The filtering circuit will either output 60 or 120 Hz.
Both frequencies are low-pitched, 60 Hz being sort of a growl. If you are European their line frequencey is 50 Hz -- close enough to 60 Hz.
As the hum does not come from the radio's power supplies then it has to be external. Now to find it. If the hum is more of a buzz and is absent at certain times of the day then might be a fluorescent lamp. Could mercury- or sodium-vapor security or street light.
If the buzzing changes at periodic intervals (like several times per minute) it might be from LED traffic signals. (They're starting to replace traffic signal bulbs with light emitting diodes. If you have noticed the signal lights are a lot brighter and flash on or off suddenly they probably have been changed. The light also appears to have a bunch of dots rather than one solid light brighter in the center.)
So suppose for now you need to use your battery-powered radio to try to figure out what the source of the noise is.
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