Well it hums cause it done forgot the lyrics.
Seriously,, COnverters come in two basic types, Transformer and Switching. Some are good, some not (And it does not matter which of those two types, what matters is what follows the transformer if it has one).
Transformers are made by winding two or more coils of wire over a common iron core.. This core is made up not of one solid hunk of iron but of many iron leaves, each leaf is varnished before assembly.
Over time the varnish dries, hardens and flakes away.
This allows the leaves to move, releative to each other, Which they do big time when subjected to an alternating magnetic field, as they are in a transformer.
There are basically three things you can do.
1: Ignore it (or figure out a way to muffle it)
2: Replace the transformer
3: Upgrade the converter to a switching converter.
