I had the same thing a couple years ago on my 2001 F250; couldn't figure out what it was, and it was driving me nuts. Checked for stones in the tread, etc., all the things you'd normally look for. Turned out to be low air pressure in the tire; once the correct air pressure was in the tire, the noise went away. I know it sounds crazy, but that's what happened. I remember the tech guy at Ford explained to me why that happened, but I can't recall what the explanation was. Anyway, sure can't hurt to check the tire pressure, and it'd be a simple fix!