I recently acquired a 2001 Ford Expedition. It has 73,000 on it and when it gets up to highway speed (50+ mph) there is an audible hum, almost like a real light whistle. It sounds kind of like a buzz you would hear coming from a radio speaker, but you can hear it when the radio is turned off. It is not constant, but could get annoying. Any ideas what could be causing this.

Thank you,
Jeremy