Hi there,
While driving on highway, if you depress the gas pedal, does the sound quits?
I remember about a similar problem found on Ford Explorer 2003+ and it was from the catalytic converter. I'm not telling this is your problem because we're talking about two different cars (D's are better

) but at least I have seen two Explorers with this same problem. It was corrected after the part got replaced.
Hope this help.
Regards,