Loud Knock from Drive Train, I Fear the Worst

[FL911SC]

Greetings to all. This is one of those problems that we all love to hate. It is a sound coming from the car which is loud enough that I fear it is a major problem. Because it is a loud noise coming from the drive train, if it is not a major issue now, it surely will be.

First the problem: It started last week on the drive home from work when I was in stop and go traffic. In first or second gear I would run the car up to > 4 thousand RPM and then left off the pedal. This causes the car to slow down and this is when I would hear a loud KNOCK. It is more of a knock should than a thump. I don’t hear the knock when I am accelerating. I can make the noise happened consistently by repeating the process above. Additionally, when I got home and put the car in reverse to park it, as I gradually stated backing, it happened again. This is inconsistent with the above in that it is a gradual start, but it is a change in direction since I am reversing.
In summary, I have a loud knock coming from under the car when the drive train first goes under stress like letting off the throttle at a high RPM for the first time and when I start backing up after moving forward.

This weekend, with the car on stands, I retorqued the engine mounts, transmission mounts and all 24 axel bolts. I shook all four wheels feeling and listening for play or knocks. I visually inspected the suspension components even though I don’t think this is where the noise is coming.
I drove the car in the driveway with my step son (former mechanic) standing off to the side so he could hear the knock. He is sure it is coming from the engine/transaxle area and not related to suspension. This is consistent with the conditions used to make the noise.
With the car in neutral or in gear I can turn the wheels without hearing the knock, but when it is in gear it seems like there is too much play in the ‘feel’ as the wheel engages the transmission. That said I have zero idea of what that is supposed to feel like.

The current thought is that the fly wheel has possibly come lose or worse, there is a problem with the differential.

I am looking for any thoughts or things I can check before having to drop the engine.

About the car: This is a 1979 911SC Targa with 83K miles. The drive train is stock. I had the motor out two summers ago at which time I had a shop replace the engine’s main seal along with turning the fly wheel and replacing the clutch. I have also had the CVs professionally rebuilt at the time.
Lastly, in January I started auto-crossing. I know this puts a huge amount of strain on the drive train since the course is usually 2 gear and jumping on and off the throttle.

Any thoughts or help is appreciated.
Thank you in advance for taking the time to read all of this.

~Keith

[toddu]

CV joints?

Todd

[Uncle Enzo]

I am thinking CVs. I had the exact same symptoms and even the same reaction. Its a super easy DIY and worth taking a look.

[FL911SC]

Thanks Todd and Enzo.
I can't rule out anything, but here is are thoughts on the CV joints...
They have less than 1000 miles (probably closer to 500 miles but I'd have to check my log book) since they were rebuilt. I did check the torque on all the bolts, and with the car on jack stands I can turns the wheels without hearing or feeling anything out of the ordinary.

~Keith

[FL911SC]

My mind is obsessing over this while I try to get some work done.
So my question is, is there a way which I could check for play in the CVs? Could I lift the car on to jack stands, put the car in first gear, block one wheel (I do not have limited slip) and try to turn the other forward (clockwise) to check for play? Would that work? How much play, if any, should I expect?

Thanks again!

~Keith

[FL911SC]

It has been a while since I posted this question, but I got the issue resoved this weekend wanted to close it out.
The problem was the CVs as was suggested. I am still having a hard time accepting that such a loud bang/knock was coming from such a small amount of play in the CVs, but after replacing the entire axle and CV assembly on both sides, the noise is gone.
I did remove the axles and turned them in to the shop for cleaning and inspection. I was shown the wear and could see the problem, but when all the play was added up, it was still <2 millimeter. It seems that was enough.

I ran in an auto cross this Sunday if even after that type of stressing, the noise is non-existent so I am satisfied the CVs were the cause.
Thank you again for those who responded.
~Keith

[ErVikingo]

Keith,

CV's will make a lot of noise when dry or going bad as you learned. For the future, the other thing that could make that kind of noise is the R/P if backspace is not correct.

Invest in high quality grease, it will pay for itself in no time....!

[Smoove1010]

Did you notice any change in the feel of the car with new axles and joints?

If it's not too much trouble and if the parts are still available, it might be helpful to others if you could post some pics of the CV joint parts - the ball bearings, the races, and the cages. It would be interesting to see what degree of wear caused this issue.

Glad to see that you got to the bottom of this.

GK

[FL911SC]

Originally Posted by Smoove1010

Did you notice any change in the feel of the car with new axles and joints?

If it's not too much trouble and if the parts are still available, it might be helpful to others if you could post some pics of the CV joint parts - the ball bearings, the races, and the cages. It would be interesting to see what degree of wear caused this issue.

Glad to see that you got to the bottom of this.

GK

I didn't really notice any change in the feel of the car to be honest.

As for pictures, I wish I had thought of that. I left the axles at the shop since I had no need for them and didn't think to grab any pictures.
The cages were in good shape. Most of the wear was on the "star" shaped part (sorry I don't know the proper name). I was told that is some cases these can be turned around and reused on a rebuild, however in this case they looked pretty warn. Lastly, when this star part was placed on the axles and slid in to the spot where it was normally sits, there was some play when rotated. When this part was slid out to the end of the axle, most of the play went away but there was still the smallest amount. This indicated there was some wear on the axle splines as well as a small amount within the star part.

[Uncle Enzo]

Love it! The prob is, like me, you will now by obsessed with CV joints.

[OsoMoore]

I'm glad you got this sorted out. For posterity's sake, often a CV joint issue will only be apparent when the joints have warmed up. In my case, only after a drive of 20+ miles did the noises begin.