I've narrowed it down to something in the drive train.

Run it till it lets go, that'll remove all doubt about the problem. :P

well im pretty sure its the front pump in the trans
cus i fixed the leak and filled it up and it still makes the noise when slowing down

something else wierd though
the motor makes no noise but u can hear a small knock in the exhaust

Bowhay-

Please tell us the setup & kind of truck. There could many things that make weird noises when rolling... that you do not hear when you are still. I.E; Speedometer/cable (I had this at slow speeds on a truck once), U-Joints, rotor/caliper, etc... it will help to understand more about your vechile & what is been done to it recently.

I know this "knock" noise you hear when it is in the exhaust I think.... but that is usually not actually a knock. That is typical of a less than stellar exaust system. Get rid of the glass packs if you really want to hear the exhaust clearly. I think the knock is an illusion the glasspacks are creating at a low idle/rpm.

it is an 83 chevrolet 1/2 ton with a brand new rebuilt 385
u joints are pretty new
i drained the trans and replace the filter and all seals

and just earlier today it knock just myseriously stopped

If it is intermitent, I wonder if you are getting pre-ign from timing or bad vacumn advance... you might consider cross ign between plug wires or a simple ign issue as well that might create that knock at certain RPM.