transmission on T660

hi.
i have situation here. i have 2010 T660 , when it is parked for 1-2 days and i start driving it everything is normal but after 3-4 hours of driving ( when trailer is loaded ) i get some weird noise on 1-2-3 gear. for example if i stop and go on traffic light i can hear some noise ( its transmission for sure) but then noise goes away. cant really explain what kind of noise is that but its similar to when on manual transmission you try to put on gear and its not going on that gear.
i went to dealer , they checked on the computer no error. drove it and couldnt hear the noise ( truck was parked for couple of days before they checked it ).

does anyone had similar situation , what can it be ? is there way to test tranny fluid like you do for the engine at speedco ?
thanks in advance

 

Its automatic eaton fuller.
413000 miles on it. Bought it few weeks ago from arrow .
in the beginning everything was smooth , now noise ( specially when have loaded trailer) nad today it was jerking on 2,3,4,5,gears and then smooth again. I m assuming it is clutch issue . Will take to shop tomorrow or monday to check the clutch.