how to cure noise/static sound

i have a 98 pontiac grand prix, the whole audio system is stock.  when the music is low and or radio is off i can hear noise or interferrance or what sounds like static.  how do i diagnose this and fix it? any help much appreciated.

Are you sure the noise is coming out of the speakers?

 In spite of what you have been told so far, since you have a stock GM stereo you can be experiencing a "phantom" sound of sorts. A great many GM products use the stereo speakers to let you hear door chimes, turn signal activation, lights-on warning, ect. This "noise" isn't very loud, but it is present with the engine off and the switch on, and it IS comming from the speakers (or Speaker). It is simply a GM thing, because they connect so dang much to the stock radio. And this doesn't dissappear when you replace it with an after market h/u, when you use the GM adaptor that is recommended ( GM-LAN-4 or such) In fact, it increases if anything, so just don't turn the unit off is all the advice I can offer."That'll get ya 10 in Leavenworth,11 in Twelveworth, or 5 & 10 in Woolworth". Groucho Marx