Do all toroidal transformers hum?

Yes, it is quite normal. If you cannot hear it unless you are that close, forget about it. On big amps with modern transformers toroidal or iron core, you get humm. Many manufacturers cover transformers to reduce audible hum or soft mount them to do the same. Top quality transformers do not hum.

Hum is just 60 Hz and 120 Hz vibrations within the transformer. There is probably a subtle effect of these vibrations on the ac wave, but you are right it has little effect. Yes companies frequently buy marginal transformers to assure a price point. You get what you pay for.

Almarg, don't forget dc in the line and poor construction.