Elizabeth is right again, but you might also need to check your outlet as well. I had a situation in my house where the plugs were replaced (old two prongers to "modern" three prongers) but they didn't actually ground the outlets - hence, looked good, didn't ground a thing. Being a former builder, I rectified said stupidity (actually re-wired the outlet, since not only wasn't it properly grounded, they had the polarity reversed as well - "egads" says I), and lived hum free thereafter. I'll probably be chasing down this problem on every plug in the house for a while, but at least I've got good music to accompany me!
chas
chas