Yup, I will join the hum along chorus as well.. There are two types of converters, Transformer type which, as they get older they tend to forget the words so they hum along (Ok, that's a joke) Will explain what really happens later.
And Switching units which are much smaller, Lighter and tend not to hum so much.
here is what is happening.
The "Core" (Metal frame) of the transformer is made up of many "leaves" or thin layers of metal, as they stack 'em up they varnish them, the varnish is sticky and holds them all together so they do not vibrate or hum.
As the unit gets older and older the varnish dries out and cracks.. The result is the leaves are now free to vibrate, Hence the HUM
Home is where I park it!