I'm thinking the famous ford power steering whine. If that's the case you might as well get use to it. Mine isn't noticeable at idle unless you're under the hood, but when driving it can be heard going through town when the radio is off.
I would start with changing the power steering fluid using a turkey baster outlined in the wiki and see if that helps it. Somtimes old fluid may have moisture or dirt in it making the famous Ford growl louder. We have to use ATF in the power steering in Fords, not power steering fluid. Another source of the growl could be the a/c compressor if you're running the defroster, mine moans and groans with the a/c or defroster on, they all seem to do that. The last thing could be electrical interference from the fuel pump comming through the speakers in the back.