Thanks for the reply.
I've read up some of the websites I found on google, and it does sound like
ground loop is a very likely culprit. The thing that has me confused,
though, is that the I read ground loop occurs most often, like you said,
with a cable TV connection - because the cable provider is grounded at a
different location and that it's the resultant differing electrical
potentials that create the hum. But, my laptop and speakers are connected to
the same system.
And even more confusing - my laptop does not actually have a 'ground' plug.
It's one of the standard two-prong plugs. It's not grounded...!
That sounds to me like a classic ground loop problem.
An isolation transformer on either the laptop or the speaker power
supply should work, although it'll cost you a few bucks. You might
also be able to break the loop with some sort of isolator (e.g. a
transformer) on the audio cable, but you might have trouble finding
a suitable yet cheap one.
If you Google "ground loop" you will find myriad discussion.
I am a little surprised to hear of a laptop and speaker system behaving
that way -- I thought their power supplies generally were already
isolated. It's possible one or the other is defective. Or maybe my
diagnosis is just wrong.
Often a cable TV connection is the culprit, but you don't mention one.