Grinding noise

Ok I have a 73 cj5 that has a 4.0 and t-18 with a dana 20 t-case. I just finished all the floors bodywork and paint and got it on the road. When I drive down the road and get to 30 MPH or over I hear a grinding noise only when I let my foot off the gas or when I give it gas. I only does it for a second. It will also do it when I hit a decent size bump in the road. Also when driving down the road and I come up to a slower car and gently let my foot off the gas to slow up it will do it also but if I quick give it gas it goes away?
At first I thought it was the rear drive shaft as I have a spring over and I do not have a CV driveshaft yet.
I drove it today and I let my foot off the gas and when the noise started I quit shifted the t-case out of gear(I have a twin stick) and it went away. I also drove it down my street in front wheel drive and no rear drive shaft and the noise was not present.

So I am not sure where to go from here. It acts like the t-case has slop or it has something to do with the whole drivetrain torqueing when the throttle is applied? Any ideas?

Now that I have it on the road again I want to take care of this

 

How much lift do you have???

Sounds to me like the axle is wrapping a bit and binding the u joints on the rear end.
Might be time for the CV

High5

 

If it's a higher pitched grinding noise and follows the road speed until you disengage the drive line, it could be some play in your transfer case. My transfer case is 31 yrs old & has never been touched (barring a flush or 3). It has it's own song just as you described.

I just had to rebuild my cast iron 3-speed after 30 years, and if I'd stayed out of highly acidic cedar water, It would probably still be running w/o a rebuild. My 232 has never been rebuilt (except to replace a newer head) & is definitely old enough to smoke.

Try some Kendall rear end/trans. additive and check the results.

A binding driveshaft would click and even jump at starts if locking up then usually make typical u-joint squeals. If your (slip-yoke) driveshaft were wiping the splines, you would get a resonant frequency vibration at certain speeds. This is failry noiseless until the splines start to slip.

Don't sweat any consistent noises, just keep an ear out for changes in the noises.

gj

 

Wow, this is just amazing. Ya know three weeks ago I went through this same thing and nobody could provide any solutions. Now I have come across many cases of the same thing. Lifes a funny thing.

X2 on the binding driveshaft. I installed a set of 6* shims and the noise dissapeared and I have no vibration s up to 65mph (thats as fast I have had room to bring it up too). 53flattie used 8* shims when he installed a 3" lift on a '74.

 

Well I went out to look at the tranny bushings and torque bushings. The tranny mount looks Ok. The torque bushing has a lower one and upper one. The lower one seems tight but the upper one is so loose that I can spin it with my fingers. So I tried to tighten it up. No luck it was rusted solid. So I think I am going to replace them first with new poly mount bushings.
I looked at the drive shaft and it is not binding up. I looked at the yokes for any marks on it from binding and did not see any. I still plan on getting a CV shaft later.

 

Could be rear pinion bearing also. Unless it was quiet before you did the springover. If it is new to the springover, it's prolly driveshaft.

 

I would check your driveline angles. I had all kinds of problems when my angles were off. Remember, the centerline of your tranny/t-case must match the centerline of your pinion. Making sure these angles are the same take only a few seconds and could help make all the difference.

 

When you say torque bushing, do you mean the tranny mount at the end of the bar? You can temporarily tighten that by jamming in a few GM front end alignment shims until very tight. That will definitely affect many items when the torque load is varied.
Unless I'm reading you wrong, doesn't one bushing bolt at the bar/plate bottom & one bushing bolt at the top of the plate with the same bolt?
One end frozen, one end loose could mean a broken bolt.

gj