There are really no code references, thought NECA standards do describe preferred practices.
In the air or on the floor? How much can you lift?
The transformer itself need not be readily accessible; having to use a ladder is acceptable. The disconnect- that's a different matter.
The 'hum' is part physics, part design, part installation, and part age.
The electrical forces involved both make the transformer go 'thunk' when you power it up, and create internal vibration that get made into that 'hum.'
Many transformers are now designed with the internal parts 'floating' on cushioned mounts, as well as other components designed to reduce the hum that you hear.
When you instal the transformer, you can reduce the amount of hum that you hear by:
-Mounting on rubber foot pads;
-Making final cttachemnts through lengths of flex, rather than rigid pipe;
-Cushioning adjacent surfaces; and,
-Not installing in corners, or other places that will tend to focus, or amplify, the sound.
