Rattlin and Vibrating noise when accelerating

[hightide]

I get a vibrating, like a hum, noise that seems to be coming from under the truck. It only occurs when slowly speeding up below 2000 rpms. If i accelerate a bit faster it does not make the vibrating sound.

I towed my uncles f150 a couple of weeks back and the cruiser made a rattlin sound. The rattlin sound only hapens when under load. Anyone experience the same?

Gracias?