Clunking in the rear
Thread Starter
Racer
Joined: Sep 2004
Posts: 348
Likes: 1
From: Southwest Suburbs of Chicago Ill.
St. Jude Donor '14
Clunking in the rear
I notice when I drive really slow like 5 mph or slower my rear makes a noise. Also when it makes the noise the tires shake. I only notice it going really slow. It does not do it all the time. But it is noticable. Can it be the posi fluid or something else? The noise is like a thump. And like I said it does not do it all the time. 
Instructor
Joined: Jun 2005
Posts: 142
Likes: 15
From: Austin TX
Originally Posted by logansrun73
I notice when I drive really slow like 5 mph or slower my rear makes a noise. Also when it makes the noise the tires shake. I only notice it going really slow. It does not do it all the time. But it is noticable. Can it be the posi fluid or something else? The noise is like a thump. And like I said it does not do it all the time.
See if there is a difference when you are going as absolutely straight as you possibly can, versus turning. Mine is wicked nasty awful when trying to park it in the garage because I have to avoid the junk on both sides (lawnmower to the right, Gran Prix to the left
)
Thread Starter
Racer
Joined: Sep 2004
Posts: 348
Likes: 1
From: Southwest Suburbs of Chicago Ill.
St. Jude Donor '14
I know I need new u joints when I put it in gear it makes a thunk as well. It happens when I turn, Also after I go straight after a turn it stiil does it. But than I think it goes away for a while. I'll have to check it out.
If it is the posi fluid where do I add it at?
If it is the posi fluid where do I add it at?
Instructor
Joined: Oct 2001
Posts: 120
Likes: 1
From: Aiea / Opelousas Hi / La.
I just had sort of the same thing happen to my 78. I figured maybe the u-joints or maybe a flat spot in the tires from sitting for 2 years. It turned out to be the transmission mount. The rubber had split and was completely separated from the rest of the mount. I noticed the noise and wobble when going slow as you said and also when slowly backing up. Hope this helps.
Damian
Damian
Le Mans Master
Joined: Oct 2004
Posts: 5,269
Likes: 54
From: Orlando Florida
St. Jude Donor '05
if it's not the U-joints then it's likely the front differntial mount or/and the transmission mount... both are easy to replace and both are not expensive....
something allows too much movement, hope it's not the trailing arm bushings... these are relatively inexpensive but a PITA to replace....
something allows too much movement, hope it's not the trailing arm bushings... these are relatively inexpensive but a PITA to replace....
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Instructor
Joined: Jun 2005
Posts: 142
Likes: 15
From: Austin TX
Originally Posted by Lee H
It's the clutch packs locking and unlocking. Doing what they are supposed to do. Pretty normal on corvettes. Sometime GM posi additive helps. Sometime not. I would not worry about it.
As I said above, mine makes a horrible racket parking in the garage. I have to make a bit of an S-curve to get around the lawnmower and avoid hitting my Gran Prix. At 1 or 2 MPH it's CLUCK-cluckity-cluck-CLUCK. What I really need is to get rid of the Gran Prix or buy a bigger garage.
Cruising
Joined: Jun 2005
Posts: 11
Likes: 0
clonks, scratch and...debris
My 76 corvette started making loud ‘clonk ‘ noises in the rear when driving. 
These sounds starts after driving for 5-10 minutes. Then, the ‘clonk’ sounds are followed by a scraping sound similar to a worn brake pad scraping a steel drum or disk. I have checked the brakes and the park brakes, they are fine.
The wheel bearings and the u-joints are under suspect
and are to be replaced. There is however no play in the joints or in the bearings but I have the feeling that they have never been replaced and could have dried up. 
Twice, I had difficulties in driving the car backwards. But after driving it forward 2 feet or so, it could be backed.
I decided to have the differential oil replaced after reading about similar ‘sound’ experience on this forum. The oil was inspected and was contaminated with fine golden or copper colored debris.
I decided to order a new overhauled differential. Is it too early for that? What solved the problems when you had it?
All comments welcome.
Regards, icyvette
These sounds starts after driving for 5-10 minutes. Then, the ‘clonk’ sounds are followed by a scraping sound similar to a worn brake pad scraping a steel drum or disk. I have checked the brakes and the park brakes, they are fine.
The wheel bearings and the u-joints are under suspect
and are to be replaced. There is however no play in the joints or in the bearings but I have the feeling that they have never been replaced and could have dried up. Twice, I had difficulties in driving the car backwards. But after driving it forward 2 feet or so, it could be backed.
I decided to have the differential oil replaced after reading about similar ‘sound’ experience on this forum. The oil was inspected and was contaminated with fine golden or copper colored debris.
I decided to order a new overhauled differential. Is it too early for that? What solved the problems when you had it?
All comments welcome.
Regards, icyvette
Instructor
Joined: Jun 2005
Posts: 142
Likes: 15
From: Austin TX
I don't have a problem -- there is a certain amount of noise that's very normal for a posi rear end. Unlike a traditional differential, posi delivers power to both wheels, rather than the fastest moving one. When you make turns, one wheel (the outside one ...) moves faster than the other. The clutches inside the differential have to make up for that. So in low speed turns you get this clunk-jerk-clunk-jerk as the clutch packs unlock, then lock up again.
What you just described, with all the metal pieces, is an entirely different matter. My rear end was very worn out several years back and needed to be rebuilt. There was a LOT of metal in the diff, same as what you've got. I forget what I paid for the rebuild, but there were a lot of very worn out parts which had contributed all the nice metal debris ...
Some noise is perfectly normal. It's annoying as can be, and not a great way to impress a date or your friends, but it's perfectly normal. My advice, for what it's worth, is have the cover taken off and the innards looked at. Then pull your wallet out and pay for the overhaul, because metal filings are never your friend.
What you just described, with all the metal pieces, is an entirely different matter. My rear end was very worn out several years back and needed to be rebuilt. There was a LOT of metal in the diff, same as what you've got. I forget what I paid for the rebuild, but there were a lot of very worn out parts which had contributed all the nice metal debris ...
Some noise is perfectly normal. It's annoying as can be, and not a great way to impress a date or your friends, but it's perfectly normal. My advice, for what it's worth, is have the cover taken off and the innards looked at. Then pull your wallet out and pay for the overhaul, because metal filings are never your friend.
Thread Starter
Racer
Joined: Sep 2004
Posts: 348
Likes: 1
From: Southwest Suburbs of Chicago Ill.
St. Jude Donor '14
Thanks every one I'll try one thing at a time starting with the differential fluid. But how do I change it. Do i siphon whats left thru the hole on the left side top? Or is there something on the other side that might be easier? Thanks for all the advice! Also were can I get a picture of vacuum line routing at. The car is a 73 L-48 with the smog pump removed. Thanks in advance.
Heel & Toe
Joined: Oct 2003
Posts: 20
Likes: 0
From: Oak Ridges Ontatio
I would check the yokes coming out of the diff. Sounds like one of them is ready to let go. Why not just take the cover off the diff. to change the oil and have a good look at both yokes while your at it.
Tech Contributor
Joined: Aug 1999
Posts: 15,175
Likes: 3,990
From: Connecticut, USA
I would look at the snubber bushing first, then the u-joints. The Posi,if setup with the 4 springs still, will require anywhere from 30-70 ft/lb to slip so it should not be banging in a straight direction. Turning corners the clutches slip under the spring pressure and chatter if "dry".They'll never be dry because you should have 90 wt gear oil in there. The additive gets in between the clutches to let them slip. Common with Eaton posi's. I don't think this is your problem. BTW the Posi's can be tuned to eliminate the chatter.
Gary
Gary
Instructor
Joined: Jun 2005
Posts: 142
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From: Austin TX
Originally Posted by rihwoods
At 1 or 2 MPH it's CLUCK-cluckity-cluck-CLUCK.
Sounds...like an automatic...???
Sounds...like an automatic...???
Race Director
Joined: Feb 2005
Posts: 16,100
Likes: 17
Originally Posted by tallgirl
If it does that in other than straight line driving, it sounds like the way I'd describe my posi at that speed.
Instructor
Joined: Jun 2005
Posts: 142
Likes: 15
From: Austin TX
Originally Posted by rihwoods
Have not heard mine ...yet...but have 4 speed...get nothing in straight line or turning...new u-joints,though..posi good..I think..have driven a few automatics...different sounds back there...just my opinion...