vince vette 2

June 3, 2019 - To see what I've found since starting this post in almost a year ago go to the end of the thread

Have been taking my 80' out for some rear end break in drives. The history is as follows:

Bought car new
Drove fairly regularly until 2004 (about 90,000 miles)
Then it sat essentially until last fall
Replaced entire brake system, had steering control valve and cylinder rebuild, radiator serviced, other odds and ends.
Took out for some short drives in the fall
Heard a knocking in the rear. This was not new having developed in the the last few years I had driven the car semi-regularly (late 90's early 2000's)
Disassembled most of rear suspension with the following done:
- Replaced original steel leaf with composite spring
- Replaced shocks - now QA1 single adjustable
- Replaced most bushings strut bushings which were totally shot with almost no rubber left
- Replaced batwing bushings
- Sent rear end to Bair's for rebuild and swap to 3.54 gears
Replaced all u-joints
Re-assembled everything
Adjusted the ride height
Still have the knocking sound - might even be worse

I wouldn't call it random but it isn't consistent either. It most often happens right after letting off the throttle at which time it usually is regular in timing and loudness for 3 or 4 knocks and then it gets irregular again in timing and sound.

I always figured it was the rear end as it first started lightly right after I had a shop replace the seals. Then seeing the strut bushings I figured it could have been them. But now with the rear end rebuilt by folks who know what they are doing and the strut bushings replaced, these are ruled out. The only major item not worked on is the trailing arms. I know, with everything else pulled out why didn't I do them also? The usual answer - more time and money than I felt like spending right then. That aside, would anything in the trailing arms cause this type of sound.

I should also note I get a lot of gear whine when let off the throttle over over 30 mph. I've spoken with Bair's who advised to drive it some more as I have only about 30 miles on it. But the knocking is now concerning me to the point that I don't want to drive it until the cause makes itself obvious by leaving a trailing arm on the highway.

Advice is appreciated.

DaveL82

Chased an issue after not owning my 80 long that I thought was a rear end noise. Took sometime but found the front passenger side motor mount was broken. This cause fan to sometimes hit hit the shroud and make a knocking sound.

vince vette 2

Interesting. I'll take a look at it.

OMF

Also have a look at your exhaust system, it may be contacting the frame as you let off and sounding like a knock.....

vince vette 2

Good thought too. Especially as it actually seems louder than before I did all the work. I did shake the mufflers and didn't hear anything. But I'll look further up.

ignatz

Since you had the rear end out, you probably replaced and correctly tightened the differential snubber bushing - right?

vince vette 2

To the best of my knowledge it is in correctly. NOS rubber snubber between the housing mounting tang and the cross member with a poly bushing on the top of the cross member. Original bolt and washers through it all, torqued to 65 ft-lb.

vince vette 2

Haven't checked the motor mounts yet - too many folks over today. But did have time to look around the exhaust. One muffler tail pipe was almost touching the frame. I rotated it to get some distance. Took another drive. Still had the same issue. The most likely time for it so sound off is if I come off the throttle quickly at lower speeds. It then starts with a random frequency and then as it slows usually hits a cadence for 4 or 5 knocks before going random and stopping. Might indicate something out of balance or it could be something is loose and when the speed it right it resonates. I'll have to keep at it.

TimAT

Back in my dealer days, (daze?), I built a lot of rear axles. If it had a whine on either the coast or drive side right off the bat, it was not going to get better.

vince vette 2

Not something I want to hear. But thanks for prepping me.

GTR1999

Vince,
I have heard guys say for years the gears will break in and quiet down. Rarely is that the case. When I replaced my 308's years ago there was a bad gear whine in them. The pattern and setup was perfect, the gears were just bad. I wanted to see if they would change over use, I tested some gear additives, drove it 800 miles but there was no change. I had to drop the diff again and replace the new gears with another set - set up the same way and they are quiet. I had another set whine, this time it was a set of USA Richmonds and they were terrible and I have set up plenty of Richmonds. I replaced those with Tom's and they were dead quiet. There may some gear noise, in fact the setup sheets in Tom's gears mentions it, but a whine off loading is most likely setup. Did you get a pattern picture of the gears? I provide pictures on all my builds but I don't know if anyone else does/

Try driving and see but I would be surprised if the whine gets much better but you never know. The diff should otherwise perform fine.

vince vette 2

Gary, thanks for the input on this. I was hoping you wold weigh in. I didn't get photo of the pattern, but since it's a Dana 44, it was returned opened and to my inexperienced eye it looked OK. I'll just have to keep plugging at it.

vince vette 2

I sent a recording of the noise to Bair's last night. They called by 9 AM this morning. They agree it's not right, and suggested I enjoy the summer driving as the issue will not cause any damage. Then I can get it back to them this fall to be corrected. Knowing their high level of experience and reputation, I suspect just a one off issue with the gear set itself. Guess I can take the trailing arms out too then.

scrappy76

Don't forget about the transmission mount too. And look in your compartment trays behind the seats, make sure something isn't loose banging around.
I am not sure if you have done this, but go somewhere like an empty parking lot, a large one. I know this is not exactly safe but I have done it and found noises easier this way. start going slow and recreate the noise. either you or a passenger open the door and put you head down where you can grasp the noise better. You might be able to pinpoint it this way.

vince vette 2

I'll check the tranny. It should be tight as I did pull the bolts a couple months ago when I had delusions of pulling the cross member and then the tail housing to fish out a lost speedo gear. But it became obvious that contrary to what would seem to be normal, the cross member wasn't going to drop out without potentially damaging the fuel lines and/or brake line. So I bolted everything back up. To my knowledge I retorqued the tranny mount bolts, but when in an aggravated state of mind things get missed.

I've also taken everything out of the back of the car, even removing the plastic storage bin liner to eliminate as many potential sources as possible. I'll find a nice parking lot this weekend and see if I can at least localize the source.

DUB

I seriously doubt it is in the right motor mount due to if it were bad...the noise would come on under acceleration and the engine would rotate UP and cause the fan to tap on the fan shroud. So..the noise would not come on when you lift off the gas pedal.

I assume you do not have the spare tire carrier installed.

The only way I can see the trailing arms causing this sound...even though I have not heard the 'sound' is if the rubber in the bushing is totally shot..

The rear wheel bearing and do it if they are bad enough and thus possibly also cause the parking brake shoes to have an issue if any if their components are broken and the assembly is not correct.

A cracked pulley or flex plate can cause a sound but that will be there even with the engine running and the car not moving.

DUB

vince vette 2

You are correct, the spare carrier is fully removed, even the top cover that was riveted to the gas tank support. I did rule out the motor mount as I saw no issue with the fan shroud and as you note this is happening primarily on deceleration. I also moved the tail pipe on the right muffler to give it a little clearance from the frame. It was not nearly as close to the frame as the left was and which I moved previously. But I've seen some movement of parts in vibration that is much more than one would think possible, so it's worth considering. Time and weather permitting I'll take it out this evening or tomorrow. If it's still there then I'll look to see there there is some way for me to stiffen the exhaust to see if that changes it in any way. If not, then I'll have to look at rotating items to find things that go bump in night.

On the parking brakes, shoes, springs, and the adjuster were changed during the brake job - would it have killed GM to put a little bigger hole in the spindle flange to work through?! But since this noise was there before that work, I think it's unlikely. If it comes to this, any thoughts on getting it up on the quick lift, pulling the tires, supporting the trailing arms at ride height and running it like that, obviously without any very rapid throttle changes?

vince vette 2

A little more info for the investigators. Pulled everything out of the back. Still heard it.
- It is worst when slowing down off throttle whether in gear or neutral
- It is worst when going through certain speeds, typically around 15 to 20 and under 10
- Brakes don't seem to affect it, but I'm not completely certain
- I checked for play in the wheels. 3 to 9 o'clock there is none. 12 to 6 there is some. Measuring at the edge of the tire rim (15") there's about 0.020" total, i.e. 0.010 each way. I don't see any movement in struts or half shafts when I do this. It's the same on both sides, driver/passenger. Car has a little over 90,000 miles
- I did notice when pulling it in the garage that when riding the brakes I do get a light tapping which is very consistent which to me would indicate that is likely exhaust somewhere. But it could be unrelated.

That's about it in case it stirs any thoughts. I tried to get a recording, but my phone decided to die. Maybe later today.

garrettb

This thread has me curious, because we may have similar noises coming from our cars. I didn't think anything of mine though, because it's so quiet. With the radio on, it's barely audible. How loud is your knocking? If you can get an audio clip, I'd be interested in hearing it! FYI, when my exhaust was rubbing, it sounded more like a squeak than a knock. A simple readjustment solved the issue and that noise disappeared.

vince vette 2

I first noticed the sound when it was very light. In fact, most people didn't even hear it when I pointed it out. It got worse over time. I first noticed it after seals were replaced in the rear end. I assumed it was something they did, but it was such a minor noise it was hard to trace it to anything. Now, I'm foggy on the details since as I mentioned earlier that was at least 24 years ago. I drove the car until 2004. The noise got progressively worse. Since I thought it was something in the rear end and I wanted to replace the gears anyway, I didn't sweat it. Last November I finally got the car on the road for a few short runs and the sound was still there. Then did all the rear suspension work except the trailing arms as I didn't think there was an issue, and of course the rear end rebuild and was looking forward to the elimination of the knock. Needless to say, I was pretty depressed hearing it still there.

I should get a good recording later today and I'll post it.