We used to put the transformer in a pot of varnish/shellac/etc and stick it in a bell jar attached to a vacuum pump - just one of the venturi types that attach to a tap (faucet).
Keep the vacuum going until no bubbles rise to the top, every little nook and cranny will be full of the stuff.
Huh. I do the same thing with Vodka and Bananas.
Does it stop them humming? (Are you thinking what I'm thinking B1? I think I am B2)
Dunno, after the first Banana I hum myself, and yes B2 I think I'm thinking what you're thinking.
On topic though, vacuum impregnation with wax is another method that works pretty well. As with all of the vacuum impregnation techniques you do have to manage the vac level so you don't boil your encapsulant. I've done some pretty bad damage to epoxy and 2 part elastomers by getting careless with the vac and boiling it. So much so I built a vacuum controller so I can dial in the required vacuum and a set of solenoids makes sure it stays where it is supposed to. Helps a lot for degassing, vacuum moulding and vacuum filtering sensitive liquids. (plus bananas & watermelons). Vacuum is also a wicked way to get a great steak marinade done in minutes rather than hours, and a great way to cheap red wine (at room temperature) into quite a drinkable brandy / lawnmower fuel.
Don't use the same pot you use for epoxy/wax to do the food or grog, and don't use a fragile container. A vacuum implosion can do a *lot* of damage, make a lot of noise and upset the missus.
RS sell nice little air/venturi vacuum devices for a couple of 10's of $$. So you don't need a machanical vac pump to do it either.