Knocking sound when accelerating

[ecurie]

I just finished my first classic tour after the body restoration of my '79 Shadow.
The car ran well except for a few problems. One of those however seems to be somewhat of a mistery to me.
When pulling away (even relatively slowly) from standstill or accelerating hard there is a distinct knocking sound coming from the rear left corner. I have to feather (pull away real slowly) the accelerator to drive "around" the problem. Once I do that the knocking dissapears and will only come back after i've stopped and pulled away again or really floored it.
I can also accelerate "through" the problem as the knocking will dissapear if I keep accelerating and I can than release the throttle without any knocking coming back.
There are no strange sounds coming from the diff, the car handles well, brakes well and sits level.
The car seems to have developped this problem during the 11 months it was immobile while having its body restored as it didn't suffer from this before.
I removed the left driveshaft (from diff to left wheel) for inspection, but there doesn't seem to be any indication of something rubbing or other malfunctions. With the driveshaft removed I can easily turn the rear hub, but that also seems perfectly normal. When looking at the side opening of the diff, where the driveshaft goes in, everything seems to be in order too.
I also checked the exhaust in case it touched the bodywork but all seems to be fine.

Could it be that the propshaft (from gearbox to differential) moves slightly backwards when driving off or accelerating hard and touches the floorpan or rear crossmember ?

Any suggestions are welcome.

Benoit




I'd appreciate any suggestion as to what to look for next.

[rich.]

is it a rolls royce? could it be a servant locked in the boot

[ecurie]

It's a Rolls Royce indeed, but alas no servant in the boot.

[Wicksy]

Not familiar with the Shadow rear suspension, I believe it's based on Citroen self leveling design but as a general pricipal checking all the rubber bushes for deterioation or drying out would be sensible. If it has conventional shockers or an anti roll bar, the mounting bushes and drop links are usual suspects or the shocker itself.
The fact that it has developed during a period out of use tends to bear out the above

[ecurie]

Hi Wicksy,

I've checked all bushes and mountings but they seem to be OK. Also, the knocking only occurs when pulling away (even slowly) or accelerating hard. Once up to speed, there is no knocking sound whatsoever, even when driving over really bumpy roads. It seems therefore to happen only when there is a high torque in the propshaft. Could dried out or worn universal joints cause this and would they knock so loudly ?

Benoit

[JPB]

Yes, and so could a driveshaft joint on the left as it's that side where you're hearing the noise.
I believe that these use pot joints on the outers so they're pretty tough, but they're where I'd have a look to start with.

[ecurie]

I found this clip on YouTube and this car seems to have exactly the same pronlem as mine with the exception that I can accelerate "through" it.
The noise certainly seems to be very similar.

http://www.youtube.com/watch?v=25B4SFZWDRs

[Wicksy]

just watched the video and his noise sounds very rotational i.e. shaft, coupling or loose mount allowing resonance.
Like has been said by JPB, check out the drive shaft first and then the prop shaft - on the prop, the joints should have no perceptable movement when loaded axially but there would have to be considerable wear, 1,0mm at least to give you a loud knock like this, I would have thought