Noise When in Neutral

[threeonthetree]

Start off i have a 350 mated with a sm465 transmission and a np208 transfer.
axles are dana up front and gm corp 12 out back 1/2 ton

i just got the truck back on the road. today was the first real test drive.

everything is great off the line, shifts smooth engages good.

but as soon as i start to decelarate it starts to make a grinding noise, if i put it in neutral it makes it more and if i push the clutch in it doesnt go away.
espcially does it when i put the truck in neutral down hill if i put it back into gear it goes away right away.

what could this noise be?

all fluids are topped off with fresh oil.