Grinding when I let off the accelerator

Having an issue with my newly acquired '88 D-90. It's real smooth running until I'm in third/fourth/fifth gear and let off the accelerator. It does not have an issue at lower speeds and doesn't seem to matter if I'm going straight or around a corner. If I step back on the gas and accelerate the grinding sound goes away. It's not an engine noise- sounds like transmission, transfer case, or axles, but I can't really put my hand on it. Any ideas?

Is it a sound like an angle grinder on steel or a loud-ish rubbing type of noise?
If it's a steel on steel-type noise, I'd be inclined to chech the brake shoes, both on the hand brake and on your rear axle.
Then come back and let us know your progress.

All the backlash in the drivetrain can cause this and only on lift off.
My 85 90 does it but it depends on what the noise sounds like and only you can hear it.

UJ's ... and what 'the others' have said ..

A perennial problem that causes metallic 'pinging' and if it's bad, grinding noises ...

Axeboy said:

Having an issue with my newly acquired '88 D-90. It's real smooth running until I'm in third/fourth/fifth gear and let off the accelerator. It does not have an issue at lower speeds and doesn't seem to matter if I'm going straight or around a corner. If I step back on the gas and accelerate the grinding sound goes away. It's not an engine noise- sounds like transmission, transfer case, or axles, but I can't really put my hand on it. Any ideas?

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the cause of that is often diff pinion bearings

Thanks for all the suggestions. I put her up on ramps today and discovered one loose and one missing bolt on the front driveshaft universal that connects to the transfer case. Replaced the bolt and retorqued. A bit better, but still lots of noise/vibration when I decelerate from 3rd, 4th, or 5th. Thinking U joints or diff pinion bearings as suggested. How do I troubleshoot from here? Thanks for being patient with the newbie.

Checking the uj’s on the propshaft is very easy. You are just looking at the condition and checking for any play. If the look dry/visible rust dust they will need replacing. If there is any movement either side to side across the yoke or rotatianally between the two parts of the prop they will need replacing. If you cannot find either I would suggest filling them with grease and seeing if it makes a differance. If it does then they are non the way out as well.
I am not sure on the diff pinion bearings but would guess you are checking for any lateral movement on on the pinion showing the bearings are worn. But I’m sure someone will be along with a more detailed way to check.

While you are under there check the oil levels on the main and transfer boxes and both diffs. If its new to you and you don't have the service history then a full oil change on everything won't go amis (as will a cam belt change). If you get that special silver/metallic oil coming out of one of the boxes or a diff when you drain it then that may point you in the right direction.

I think I may have discovered the same or simular noise on my 300tdi. I will check the diff thursday and see. if you solve it let me know what you've done.

weirdly comforting to know someone else is in the same place as you. Not meaning to sound cruel.

When mine had these symptoms it turned out to be the rear diff.

Alfieslandy said:

When mine had these symptoms it turned out to be the rear diff.

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what did you do? rebuild it?

Replaced it with a rebuilt one that I bought. The pinion gear in the old one was totally shot.... looked like something had got between it and the teeth of the crown wheel and gouged out a pretty hefty score in the pinion teeth.

hmmm. I have a week before i need to head to Europe for a month. Not really time to get a new one, and not really in the mood to remove and rebuild one just to discover more issues. I think I might just lock the centre diff and remove the front prop-shaft.

Out of sight, out of mind.