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My 1986 coach drivetrain whines when I let off the gas or when going downhill. Should I be concerned?

[HoosierDaddy]

Check rear drive axle lube first. Check pinion bearing.

[Richard]

What Dean said.

To add to that, both Newells I have owned have had a bit of whine in the differential.

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I believe I have a serious problem. I crawled under and Checked the fluid level it was ok. I grabbed the driveline And there is considerable play in the input shaft to the differential. U-joints appear tight.

[Johnnyboy]

I sure you are aware or if you did not know the differential gears in your bus were changed by the previous owner when they first bought the coach.

Fast forward thinking, the rear end is not the same original with many proven miles. You may only have 3,000 miles on that rebuild rear end. These days I question the quality of repair work. I have had issues myself from a supposedly professional shop. 

My point. Your symptoms do sound like rear end noise, and it could be that your rear end was not setup correctly. I have also seen ride height be incorrectly set causing the drive shaft to be to many degrees out of line. 

Just some thoughts to consider in your investigation.

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Turns out the u-joint is the issue. They found the yoke nut loose. Putting it back together to test drive it. 
Sounds like you know a bit about coach  #121? I did find a receipt from 2016 where the rear end was rebuilt, rebuilt and new gear’s installed. 
Thanks so much for the help and insights.