2008 Dodge Ram humming noise

I have a low grade humming noise coming from the truck while driving it when the truck is idle it sounds fine I have even acccelorated the rpms while idle and still no humming noise but once I get over 10mph I can hear it and it increases the faster I go but when I come to a stop light you can hear it decrease and stop once the truck is idle I can't tell whether its coming from the front or the back and ideas thanks
Michael P