I just purchased a 89 pu and it already has the dual t case setup sometimes when I am goin down the road and coasting in gear I here a little gear noise not really grinding just like the gears may not be wore in the dual setup has not been run much since installed anyone ever had this problem

 

It would not hurt to check the fluid levels, dual cases share the same fluid, make sure it is up to the level of the rear fill plug.
My dual cases are a little bit louder than a single case. Some say a 4.7 gear set are a little louder too.
Especially if the interior has been gutted.

**but with out actually hearing it hard to say..