Do all toroidal transformers hum?

The reason for the hum is what is called magnetostriction. You can read about it here.

It may also be sensitive to the tightness of the transformer mounting screws, and to the design of the mounting arrangement, which might influence the degree to which the vibrations are amplified or damped by the surrounding structure.

Regards,
-- Al

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

You may also help by using either coupling or isolating devices under your amp. Although they work on opposing principals I have found both to improve the sound to a surprising degree. I have been using Audio Points from Star Sound but am now using FIM isolation devices [modified] under all components. But there are many others.

Thank You all for the great advices. Just want to make sure that the hum is not going to damage anything (I'm new to this stuff,so please pardon my ignorance). The DK does have covers on it's transformers.

I would not worry about hum that is audible only two inches from the top of an amp. Virtually any toroidal transformer, even of the best quality, will buzz to some degree if the DC level on the AC power line is high enough.