strange noise at low rpm

[myfirsttruck]

Just bought brand new F-350 Lariat. It's my first truck and I just love it. However, at low rpm just barely touching the throttle at 70km/h I occassionally hear a very noticeable growling vibrating noise (hard to explain). I took it to the dealer and took the mechanic for a ride and after ten minutes was able to make the truck produce the noise which he agreed was not normal. He spent three hours to tell me he couldn't find the problem and to bring back in a month if still bothering me. I'm just hoping that maybe someone may have some experience with the same problem. It seems to happen in fifth gear when just "lugging along" with just the right amont of throttle. As soon as throttle is slightly increased noise is gone. Never thought i would buy a new truck never mind a beauty like this one so im really hoping they will fix it for me. The noise seems to come from somewhere under the cab. Thank you.