Your charger contains a fairly large transformer. Transformers are made with layers of iron laminations. After they are mated with the windings, the assembly is held together with (usually) two machine screws. Finally the whole thing is dipped into varnish.
Usually, the screws and varnish will hold the laminations tightly together. Occasionally, things loosen up and the laminations will vibrate against each other, producing a humming or buzzing sound.
Generally, this is fairly harmless. It is just annoying. If it is out of warrantee, and you feel up to it, you can sometimes open the case and tighten the screws.
Retired electronics engineer. Avid paddler & birder.
2011 Silverado 2500HD, diesel, 4x4,crew cab, 8' bed
Palomino Puma 253FBS (27' 5er) & '94 19' Class B