Need some input on noise

[augiep9]

I have a m20 4 speed in my c10. It was freshly rebuilt. Put the truck back on the road last August and it has been making this sound the whole time. When driving on the highway and it is decelerating between 70 and 60 it making a whining sound. When I push in the clutch it goes away but it it's still in between 70 and 60 when I release the clutch it comes back. Accelerating is fine and decelerating under 60 is fine. It's got me stumped. Any help would be much appreciated. Thanks