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Under acceleration, especially in first gear, a rapid machine gun staccato knocking can be heard coming from the rear. The rapid staccato knocking can also be felt on the rounded rear floor hump that sits above the pumpkin.
Is this an indicator of body sag or might there be something else going on?
Hockey pucks were added under the floor above the cross member. There was no change in the staccato knocking from the rear.
If the rapping sound is rapid probably a drive shaft U joint hitting the body -- put the car on a lift and look at the fiberglass just above the diff U joint - if that is the problem you should see fresh scuff marks on the fiberglass.
The sound happens under acceleration because that is when the greatest amount of torque is passing through the drive line - it is twisting the diff upward just a tad - enough to make contact between the U joint and body. Just a guess.
If the rapping sound is rapid probably a drive shaft U joint hitting the body -- put the car on a lift and look at the fiberglass just above the diff U joint - if that is the problem you should see fresh scuff marks on the fiberglass.
The sound happens under acceleration because that is when the greatest amount of torque is passing through the drive line - it is twisting the diff upward just a tad - enough to make contact between the U joint and body. Just a guess.
To add to this your diff front mount may be shot allowing the front of the diff to pivot upward.
I just, last Winter, replaced my Snubber bushing set on my 67 and also inserted 2(two) hard rubber 3/4" spacers (2" x 3") one on each side of the center cross member (exhaust) to raise my floor panel an added 3/8". That will also raise the floor panel at the front of the differential, also. You have a few things to check.......Per a JohnZ post: floor panel clearance at the center crossmember should be 3/4".
I was under the car yesterday. All bolts and nuts were checked for tightness, all good. The front pumpkin metal bracket support and snubber bushings were checked, also good.
All U-joints checked out fine, no looseness in these or the stub axles.
No witness marks on fiberglass were seen where a U-joint could make contact.
Installing the hockey pucks above the center cross member raised the floor pan substantially. I believe the gap between floor pan and top of the cross member was 3/4 inch or close to that after the hockey pucks were installed.
The ‘staccato’ only occurs when the rear end squats down under torque. The staccato noise level can be modulated, so to speak…..more throttle hence torque raises staccato noise level.
Thinking to get under the car again and if possible feel for and try to gauge clearance gap between body and top of pumpkin.
If you are hearing what you describe, you are rubbing, but just not seeing the mark, which can be like a pencil line. Mine did it too until I added the Dick Guldstrand rear end locating kit.
Check your leaf spring carefully for a cracked / broken leaf. The loss of 1 leaf's worth of tension might be enough to allow the rear end to squat during hard acceleration / max torque. It obviously would not be the main - largest leaf - if that one broke you would know it. Difficult to inspect each leaf without removing - not suggesting that - but look along the edge of each of the secondary leafs - on both sides for a hair line crack - if the car's stance seems correct - it wo9uld more likely be one of the shorter secondary leafs. One more thing - with the weight of the car on the rear suspension check the torque value / tightness of the 4 bolts that secure the leaf spring plate - might be loose but that is a long shot.
John - regarding those pinion snubber bushings, the way the cars are configured, you can really only inspect the lower bushing. It's impossible to visually inspect the upper bushing and trying to do it by feel leaves a lot to be desired.
Check out the pinion snubber bushing set I removed from my 65 L78 Roadster. At first glance the lower bushing looked pretty damned good. Got it all apart and the upper/smaller bushing was completely wasted.
Not saying this is the cause of that Rat-a-Tat sound you're hearing, just mentioning that the snubber could be 'bad' after all.
Mike T - Prescott AZ
Leaf spring appears to be ok but will double check. Snubber bushings were replaced but will check those over again. I hope to get back under the car tomorrow.
Thanks for the suggestions.
I know you said the sound appears to be coming from the rear, but have you checked the inspection plate on your trans? An inspection cover that's loose or hitting the flywheel can cause a "machine gun staccato" sound like you describe, and drive train noises can travel through the driveline. I assume you have a manual trans.
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Dennis
perplexing ratta tat
