It hums, said the customer.
I'll take a look, I said.

After a cleanup I tested it and couldn't find anything electrically wrong.

But with nothing except power connected there always seemed to be a faint
hum from the speaker.

Probably normal I thought. It doesn't seem to be the highest quality design.

But then I noticed that the bench seemed to be vibrating a little.
And when I disconnected the speaker the hum remained.

It's actually caused by the power transformer.
Pushing hard on the back of the unit by the transformer eliminates the hum.
Perhaps tightening the transformer bolts will fix it.