Transmission question?

[Squareforceone]

I have a 1990 gmc halfton 2wd with a 4.3 5 speed (1,2,3,4, overdrive) First question what is this tranny called is it the nv3500? Second, this transmission is kind of noisy and I'm not sure if it's normal or getting worn out. But when you first put it in to gear especially at lower rpms you can hear the gears meshing i don't know how to describe it I guess like rocks being flung around in the case. Anyone experienced this?