Casey and DP are right.
Just have your dealer note it in the vehicle record.
Drive it awhile and observe.
Obviously if it worsens take action.
There is some thought this is play or binding between the transmission mainshaft splines and the clutch disc splines setting up a vibration which transduces as a sound a lot like a bad mainshaft or countershaft bearing in the transmission.
That's why when you move the shifter against a syncro pack, you put enough pressure on the mainshaft to kill the rattle the mainshaft is making in the disc splines.
So far on the trucks we know of, when removed, the transmission bearings all check out OK. In the three transmission change deal, the final fix was replacing the clutch disc and plate.
Alot of work to do if it will quiet down by 3000 or even 10,000 miles.
This is why the best plan at the moment seems to be wait and listen.