Knocking / tapping sound in rear. Watch this video to hear...

[smoothy321]

Hi, hope someone can help. Watch a youtube clip to hear the noise below...

1979 automatic, just had tranny rebuilt. A couple weeks later got this clunking in the back rear right under the wheel well and battery box compartment (coincidence?). I can put my hand on the battery box and feel what seems to be a tapping sound emulating right from that area. The tranny mechanic put up on lift found nothing loose or any rub marks anywhere…. The wheel does have a little play but could a wheel bearing make this sound? The rhythm of the clunk seems to be in keeping with the speed of car but dissipates when you accelerate but resumes when off the gas and over bumps. We also discovered that the sound is only there when a passenger is inside. So its weight related. Well, the knocking sound, when driving alone, is very minimal.

Watch the clip: c3 corvette 1979 knocking Tapping sound rear end battery box compartment - YouTube

and another: https://www.youtube.com/watch?v=BtB_6CwcwMk

Thanks

 

[Antz81]

Says video has been removed.
Did you find the problem?

 

[smoothy321]

Says video has been removed.
Did you find the problem?

hm... the wrong link was in there... all set now

 

[Antz81]

Much better. Well the link anyway.

Whats the condition of the bushes for the diff mounting? (Particularly the front bush)

 

[Phillip123]

Never dealt with something like this before... Find out what it was?

 

[GTR1999]

That is the area of the 1/2 shaft. Check the u-joints for rust and play in them. They should be tight. Make sure a cap didn't come out of a joint, I have seen that happen when the clip pops out. Check for play in the bearings. loose or broken shock, loose exhaust.

 

[smoothy321]

Much better. Well the link anyway.

Whats the condition of the bushes for the diff mounting? (Particularly the front bush)

Are the attached images the bushings?


here is another video
https://www.youtube.com/watch?v=BtB_6CwcwMk

 

[smoothy321]

That is the area of the 1/2 shaft. Check the u-joints for rust and play in them. They should be tight. Make sure a cap didn't come out of a joint, I have seen that happen when the clip pops out. Check for play in the bearings. loose or broken shock, loose exhaust.

thanks, check out the new vide I added to see if you think it's the wheel bearing... I tested for play
https://www.youtube.com/watch?v=BtB_6CwcwMk

 

[GTR1999]

If your bearings were that bad you would feel them once the car is jacked up. They would be rough spinning and noisy. Play at 6-12 is typically axle endplay in the diff. Play at 3 - 9 is inline with the bearings. If you can move the spindle in by a lug stud there is too much end play in them. If the rotors have the rivets still in the hat then chances are the arms are untouched. If the rivets are gone then you know someone was in there and the rotor may be replaced.

If you jack up the car to test it do not run it in gear on stands, the rear suspension will be hanging down and bind up. You can put a deep well socket under the arms to bring them up to close to ride height and check them. I have seen the posi case broken from hanging suspension in the air and powering up.

 

[Hib Halverson]

Inspect the attachment of the front of the rear drive axle to the chassis. Remove the bracket by loosing the nut which hold the RDA's front bracket to the chassis and remove the bolts holding the bracket to the RDA. Inspect the bracket to see if the holes on its side are elongated. If they're ok, reinstall the bracket. Inspect the rubber isolator which goes between the bracket and the chassis. Replace as necessary. Make sure all bolts and nuts are tightened per the Service Manual.

 

[smoothy321]

If your bearings were that bad you would feel them once the car is jacked up. They would be rough spinning and noisy. Play at 6-12 is typically axle endplay in the diff. Play at 3 - 9 is inline with the bearings. If you can move the spindle in by a lug stud there is too much end play in them. If the rotors have the rivets still in the hat then chances are the arms are untouched. If the rivets are gone then you know someone was in there and the rotor may be replaced.

If you jack up the car to test it do not run it in gear on stands, the rear suspension will be hanging down and bind up. You can put a deep well socket under the arms to bring them up to close to ride height and check them. I have seen the posi case broken from hanging suspension in the air and powering up.

ok, thanks

 

[smoothy321]

I think I forgot to mention that I discovered THIS ONLY HAPPENS WHEN I HAVE A PASSENGER !!! It is very very minimal when I am alone in the car going over a large bump.... This sound happens in this instance because of the corvette body being compressed which briefly mimics the weight of a passenger.

 

[Mac]

Perhaps this is a bird cage issue? How rusty are your body mounts?

Mac

 

[smoothy321]

Perhaps this is a bird cage issue? How rusty are your body mounts?

Mac

Oh god, u didnt just say that! Ha... I had the transmission rebuilt last month, could that have mis aligned something I wonder...

 

[Hib Halverson]

Now, I'm really confused.

The OP says the car is a '79 but the images you posted clearly show the drive axle for a 1980-82.

 

[Antz81]

Are the attached images the bushings?
View attachment 21801View attachment 21802

here is another video
https://www.youtube.com/watch?v=BtB_6CwcwMk

The side bushes are there. I was more thing of the front bush. Have a look at the front of the diff above the drive shaft.

 

[smoothy321]

Now, I'm really confused.

The OP says the car is a '79 but the images you posted clearly show the drive axle for a 1980-82.

you're right i just discovered... title says 1979 though.... must be late model when things were changing over...

 

[smoothy321]

The side bushes are there. I was more thing of the front bush. Have a look at the front of the diff above the drive shaft.

I was forced to bring it somewhere with a lift.... guy says 2 side bushings are bad and the front diff bushing... and he says the frame is bent.... and the diff pinion seal is leaking.... as well as body mounds bad of.... all this crap causing the left shaft to slightly rub against the battery box he says.... I wonder if I should have the bushings done? He said it might fix the issue but didn't give me a price... said he wouldn't know until he started it.... What do you thing would be an accurate price for doing the three bushings and pinion leak? Thanks for your help

 

[Vettehead Mikey]

you're right i just discovered... title says 1979 though.... must be late model when things were changing over...

No '79 Corvette left the factory with an '80 and up 'batwing' style diff. Somebody possibly modified the car to adapt one and done it poorly which is the root cause of your present problems. Impossible to know how much it will cost to fix.