I will never understand women or automatics. Conversely, please tell me this noise isn’t bad

I bought a c4 auto trans for our bronco from a guy who runs a small auto repair and custom shop who said it was rebuilt. 
 

not knowing much about them I wanted to pull the pan and he asked why. I just wanted see if anything looked old or new.  He said he doesn't work like that and I, being dumb and knowing I wouldn't really even know what exactly to look for decided to take him on his word. 
 

well, that was abo a year ago and I finally got the truck together.   I will spare the sordid details but it went into gear and I was encouraged.  It seems to drive and shift ok but on decel above about 25 it makes a really loud groaning/whining sound that is speed related.  
 

shifting into neutral makes it stop. 
 

I know it's not much to go on but I've seen the hive do far more with far less. 
 

please tell me I just need to rap the 4th pan bolt on the left two times with a mallet and it will be fixed.  

If it is on decel only it may be the trans loading the rear end?  I would expect that to whine more than the trans.

Of course I dont need to tell you that in the future you should run whenever someone doesn't let you do a cursory inspection?

fluid level in park?

C4's are super easy to rebuild, and don't really need any special tools.

I say get in there, and learn that trans.

I'm going with a diff noise.  I'm not going to say there is nothing in an automatic transmission that can make a noise on coast, but I'm sure going to say a bad ring gear, or a loose Pinion bearing is way more common.

In a C4 the one way clutch is in overrun on decel in 2nd and 3rd and can make a noise.

Hmmm.  Good info....

so a bit of history: we bought it as a manual truck that hadn't been run in years.  It has larger (33"?) somewhat aggressive tires. 
got it home, got it running decent and drove it around a little bit.  It never ran great, sometimes wouldn't crank, sometimes wouldn't start.   A few incredibly loose valves (oblong guide holes, intake and exhaust valves hitting each other...), ancient gas and loose wires all were rectified. )
 

when I Say a little bit I mean probably 50 miles, 40 of which were on the freeway so not really stop and go. 
But we never had this sound.   

then my wife said she wanted it painted so I stripped it down and sent it off.  During it's a sense, I rebuilt the motor, bought this auto trans (try as I might I could not convince her to keep it a stick) and got ready for its return. 
 

I did not touch tires, ujoints or rear end. 
 

It almost sounds like tire noise but the noise is gone during accel and cruising. Assitionally, and I will double check this, but it seems not be there at low speed decel at all, but doesn't really sneak up on you. It's either there or not.  And since it shifts super smooth, I wonder if it is only happening in 2nd and/or third?
 

the rear end leaks but I made sure it was topped it off before driving for the first time and it never made this sound before. 
doesn't mean it isn't the rear end, and that is a great suggestion, but I only changed the motor and trans so I kind of suspect it is ok.   
 

I want to see how the gearing is for highway use to see if I want to change it before pulling it apart. But since it leaks bad I will definitely be pulling it out. 
 

I just remembered that I pulled both axles and replaced the axle bearings and seals during the down time.  I made sure the bearings were fully seated but I did sort of get into the rear end during the down time. 
 

Finally, there were some signs that this guy either didn't do a rebuild or was not good.  It was painted black. Not just the trans but the gunk and dirt in the crevices as well. 
It came with the adapter on the tail end and I pulled it off just to check the seal since I had bought a new one.  The seal was as hard as a rock and the bearing felt, well, weird.  So I bought a new bearing and replaced both.  
 

when I filled it, the pan was leaking as was the kickdown seal.  I added An oring to the kickdown which seems to have pretty much stopped it as did snugging the pan bolts a little more.  

so I have little faith in his "word" that it was rebuilt or rebuilt correctly.  
 

I am not scared of digging into this thing.  If the noise persists after going through the rear end, how hard is that one-way clutch to replace?  Special tools??

would it be worth trying some magic additive in the meantime? Marvel mystery oil or similar?
 

Streetwise and Asa nailed it!!  The rear end had a small leak and I was going to fix it later but it started to get real bad.  My buddy said I should just take it to a shop as he said the pinion was loose.  I said no way was it as it drove fine before and had never made this noise before the new transmission. 
 

well, he was correct - the pinion could move in and out and up and down. So much for my $30 seal hopes. 

i took the third member into the shop (which is only about 25 miles away but normally takes probably 1.5 hours each way.  But with everyone on lockdown it was only 30 mins!) and they immediately said it needed a full rebuild.  It wasn't cheap but it's right and the best part is I don't have to go back to that guy and try and get my money back! 
 

that's right, the horrible sound is gone and it drives awesome.  Except for the weak brakes but that's hopefully fixed when I put the new master cylinder in. 

Yay.