Smart Shift transmission noise @ around 60mph

So i have a 2008 cascadia i just bought used, the truck runs ok, except on 10th gear, when i accelerate, it sounds normal, but when i let off the gas pedal, it makes a weird noise, not really a grinding noise, but an unpleasant engine brake noise.

ironically, if im at 60-65mph and the use the engine brake, the sound sort of goes away.

anyone know whats going on?

its a 2008 cascadia with an automatic smart shift 10 spd.