Loud Knock

[lakeholme]

Went for a ride in a friend's 69 today.  It has a loud knocking sound from the rear of the car that happens for several seconds after it hits a large bump.  The car doesn't jerk or pull and the sound does go away.  He told me a mechanic, who I know does good work, reworked the differential a few years ago.  Sitting in the front passenger's seat, it seemed to me like the sound was coming from the rear driver's side wheel area.  We checked the lug nuts, pulled the wheel and checked the rim.  I do think he needs new shocks.  Suggestions? 

[Mike S]

Check the rubber in the front spring bushing to see if it's eroded. Or it could be a broken shock but that usually shows when even bouncing the car.

[dutch]

Does the front spring mount appear to be secured properly into the floor cavity with the 3 bolts and captured nuts?
Seems to me I heard someone else complaining about a similar noise and they found that either the formed nuts or their retainer clips had broken and things got flopping around when a big enough bump was hit or at a particular angle on one side but not at all when the other hit a similar bump..
You can't really see if all is well if the car is raised on a ramp lift where the load is pushing the front spring eyes up into the pocket - since it loads the entire spring, pocket, and attaching hardware to sometimes make it appear to be intact and normal.
Only way then is to give the bolts themselves a turn to see if they are in fact tight - and obviously if on a post style lift any looseness should be more readily apparent at least visually..
Other possibilities - spring eye bushing itself // spring mount/bushing, or u bolt problems // shock mount problem // or exhaust hanger on that side if dual exhaust..  

Good luck - curious to know what you eventually find..   Randy  

[lakeholme]

I'll let you know what he finds when he gets it up in the lift.
I know he has practically new exhaust...

[lakeholme]

It's (at least) the driver's side rear shock.
We were going to put it on a friend's lift, but then it started raining.
When you push up and down and at an angle, it pops. Also, he needs rear shocks for sure

[tom]

Long time ago (1970 something) had a customer car with thumping & vibration for first couple minutes, but only when it was cold. Always stopped before he got it to the station, about 2 -3 miles. Turned out there was water inside the tire that froze at night, but melted as the tire heated up during use. Don't expect that has anything to do with your issue, but you reminded me of it. Was a pain to figure out, but obvious when resolved.

[hotrod68]

Dumb question, but does it have bigger tires? I ran into this with my '68 with negative-offset wheels. The tires would rub and "knock" in certain conditions. Just a thought.

[lakeholme]

Long time ago (1970 something) had a customer car with thumping & vibration for first couple minutes, but only when it was cold. Always stopped before he got it to the station, about 2 -3 miles. Turned out there was water inside the tire that froze at night, but melted as the tire heated up during use. Don't expect that has anything to do with your issue, but you reminded me of it. Was a pain to figure out, but obvious when resolved.

Tom, ice in the tire??? Now that's an interesting story!  And at least a good excuse to build a heated garage... or move to Florida! 

Dumb question, but does it have bigger tires? I ran into this with my '68 with negative-offset wheels. The tires would rub and "knock" in certain conditions. Just a thought.

Hotrod, No such thing as a dumb question when trying to diagnose a car problem... 
But, no big tires.  The car is pretty well stock, and so signs of tire rub, either.

Again, shocks are definitely part of the problem, but I'm not convinced that is all of it.  Neither is he.  When he gets it on the lift, he's going to look at bolt connections and bushings.
Maybe he'll get lucky and it is just a bad shock.

Thanks to all!

[tom]

All moving options will be on the table when I finish school, late 2014 if all goes well. (sooner if all does not go well)

[dutch]

Just wanted to add that on the possibility that it has something to do with the attaching hardware (bolts and captured nuts) being broken or loose - I had this problem with my Z 2/8 and couldn't tighten up the bolts as the clips had broken allowing the nuts to work themselves loose up inside the pockets.
I sourced some bolts with a larger diameter and formed (I call them captured) nuts from a local GM dealer which were not just regular nuts spot welded to a clip, but actually press formed. They were quite expensive - what GM bit isn't? - but the larger diameter bolts still fit through the holes in the spring eye brackets and the formed nuts still squeezed into the holes in the body pocket with a bit of tweaking to result in a truly stronger than OEM deal..
Once finally together, although an expert judge may catch the subtle difference, this arrangement is a much stronger deal than the original size hardware and better than any aftermarket clip that just has regular nuts tacked on, certainly..

Randy

[lakeholme]

It turned out to be the bushings. He drove too long on worn out shocks. The divers rear was virtually gone and passengers front was cracked.