Normally this kind of noise comes from the rear end due to a less than perfect pattern;I think MOPAROFFICIAL nailed it.
But if you are absolutely sure it's coming from the trans, then trans has to come out, and apart.
I'll guess, (besides the input bearing)
one of the frontmost two gears has serious burs on it,either the input gear or the cluster-drive.
or one of the cluster washers is spinning and eating up the case (but this noise goes away after a while lol) .
or, most likely, one of the cluster washers is laying in the bottom of the case, and the cluster is moving back and forth, rubbing on the slider(s)
or possibly dropped rollers out of the position in the back of the input. There is only room for one or at most two rollers in the cavity.
or as said, the cluster dropped rollers during the install.
But for these last two,I would expect it to make noise under power as well. Same with the input bearing.
The best way to install the tailend is with the box face down on the bench, and the cluster pushed to the bottom of the case. Then you drop the tail in upside down, rotate it slightly to access the clusterpin hole, reach in and position the cluster, aligning the washers, then dropping the pin into place. Finally, lift the tail up just enough to get it off the gasket and rotate it into alignment and gently drop it down. Put at least two bolts in all the way finger tight then snugged. Now you can align the struts to the brass on the back of the input and push the input up. then if your bench hole is big enough, install the retainer. If not then you will have to lift the pig out of the hole to lay it down. This is IDK, maybe 30 pounds easier with an aluminum case and tail.lol.
Throughout this, the input has to be in position and dropped as far down as it will go. That means your bench needs a hole in it big enough to receive it. This will provide a lil clearance for the brass not to get bent if you don't get the struts engaged right off.
I prefit the tail into the empty case and check the clusterport access, sometimes you have to clearance the tail to get the pin to slide by.
Caution,
when doing it this way, the front slider likes to drop down, and the struts pop out. So pre-stretch the energizer springs to put a good grip on the struts, then go slow, and watch your fingers! If the slider drops down, DON'T PANIC. You can lift the slider up and pop the struts back in one atta time. No big deal. Watch the back slider too.I usually slip a spacer under it, and yes, I pre-stretch the energizer springs on it as well, during the rebuild.
It might seem tricky to do it this way, but only the first time. Grease and gravity will keep all the rollers in place and a lil grease on the case side of the washers will more or less keep them in position too.
