There are many types of converters: ONE is a transformer based, In this a common transformer (2 windings around a common Iron core) convert 120vac to around 10 or 20 VAC (20 is Center tapped) and then by use of some electronics convert that to around 14VDC (NOTE, Low voltages are approximate).
The other type is a switching power supply,, they can hum too but not as likely.
The iron core of the transformer is not a solid hunk of iron, it is dozens of thin metal leaves that are coated with varnish and then sandwiched together.. Over time the varnish dries out and flakes away allowing the leaves to vibrate in step with the magnetic field the coils of wire create.. The result.. HUMMMMMMMMMMMMMMMMMMMMMMMMMMMMMM.
That, at least is the theory.
The real reason it hums... It forgot the lyrics to one-note-samba. (it only humms one note after all). This is just a little samba, built upon a single note.
(Ok, Ok, this is what happens when an electroincs technician learns to sing.. Bad jokes).
Home was where I park it. but alas the.
2005 Damon Intruder 377 Alas declared a total loss
after a semi "nicked" it. Still have the radios
Kenwood TS-2000, ICOM ID-5100, ID-51A+2, ID-880 REF030C most times