my Frontier does it too. It's a low volume, kind of a high pitch frequency. I actually think it's somehting motor related. I hear it much less or not at all when windows are down.
Try this: Turn radio off and drive from a stop slowly to speed. If it's like mine it will start getting worse around 2300 rpm's at lower speeds. It will continue doing this as I drive, just tap gas to get it to around 2300 or slightly above. But, to test and see if it's motor or something else while in motion, put in neutral and rev to same rpm's you heard the noise when truck was in drive. If it make's the same exact noise I think it's just a chararestic of the motor. Possibly airbox or some sort of valve???? I'm not really sure.
I'll have soimeone rev it to around 2300 rpm while i'm looking at engine sometime to see if it does it then. Maybe I can pinpoint it.
--2006 Nissan Frontier SE--
--2003 Nissan 350Z Enthusiast--
--2005 kawasaki zx6r blue-- --2003 yamaha r6--