Hum from Mark III

[Luca79]

I have a problem with my Mark III head: it hums and when I touch the strings of the guitar, or any other metal part, I hear a "pop" and the hum goes away.
There should be something wrong with the amp, because all the other amps I have don't make any hum.

Since the chassis was redone with hardwood, maybe they missed the grounding?
What should I check to find the problem?

Thanks