Considering it is present, no mater where you tune the radio indicates that it is probably bleeding in through the radio’s IF chain. That is around 10.7mhz. It may be a sub harmonic of that frequency, around 5.35mhz or on down.
If the radio’s IF is a bit wide and the source is close, or wide, it may be the 13Mhz ISM band bleeding into it. Things like RF part heaters or
In my own honest opinion, it sounds like a diathermy machine.
I have heard some over the radio, and each brand, and type will have a different sound, and rhythm to it. They normally try to apply heat in waves to
help sooth muscles. You will often hear them bleeding over the radio when you are around a nursing home. They are popular there.
The reason it is bleeding into your radio is you have a cheaply constructed radio. It doesn’t mater how much the sticker price on it was, the radio
probably has very poor front end filtering (or none at all). And/or it has no shielding of the IF chain.
I would say you would find it if you had an HF radio and tuned through the 13mhz, 10mhz, 5mhz, and 3Mhz bands in that order.
If you don’t want to listen too it, then get a better radio.