may be a bad or worn ring and pinion gear axel brgs' or low gear oil
You may need differential fluid...check that.
rear wheel bearings are bad
Could be a bearing or a worn out crown and pinion set.
You blow into the Flute's mouth, The sound comes out the rear end.
bad wheel bearings
bad wheel bearings
replace the rear shock asorbers
ROARS
Probably your u joints or the rear end has too much slop.
To the end of your drive.
if the sound starts when you turn the key it is most likely the fuel pump, no worries If you're at 50,000 - 60,000 miles or more, could be your rear. Common problem ford won't own up to.
One would really have to hear it in order to determine for certain. Are there any particular conditions existing in which the humming occurs? Wheel bearings are a possibility.