OK, I'm asking this on behalf of a friend with no ng access so be gentle
with me if this is a really stupid question!

He's got a Quad 33/303 based system and wants to connect his laptop to it to
play mp3s. He tells me that everything works fine when the laptop is on
battery power but as soon as he plugs in the power supply, there's a loud
humming audible through the stereo speakers.

It's obviously something to do with the presence of the mains power but what
can he do to stop it?

TIA