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MG MGB Technical - Strange knocking from rear end

I need your expertise!

For the last few months I've had a soft, but persistent knocking sound from rear end of '72 B roadster when driving over a bumpy road ie all roads these days.

It sounds just like worn shock absorber bushes. So despite the links looking OK, I replaced them anyway. The shocks themselves look fine with no leaks, were removed at same time and bolted back in tightly.

All U-bolts are nice and tight and rubber pads are in good nick.

In addition to aforementioned knocking I had a harsh metal crashing sound which was a badly worn front spring eye bush that had managed to break a spring eye. So on with a good pair of spare leaf springs (no broken leaves), together with new front bushes and new shackle bushes at the other end. That cured the crashing sound, but not the knocking.

I've taken everything out of the boot. Boot lid's not loose. Knock still there.

Changed exhaust in meantime. Knock persists.

Wheels aren't touching bodywork.

UJ nuts are tight, and doesn't appear to be any play in UJs.

Can't see any other witness marks from anything touching.

Doesn't seem to be any adverse handling effects.

What haven't I thought of?

Thanks in anticipation.

Pete
P A Allen

Pete: I had a knocking or clunking in the FRONT end which turned out to be the 8 cushion pads mounted on the front cross member.

I would go over the same bumps if possible and listen if it changes pitch, or intensity as your speed and angle of approach to the bump varies.

Also, how are your fuel tank mounting straps? Does the noise change as the the tank empies vs. full?

Hope this gives you some thoughts

Cheers

Gary

79 MGB
gary hansen

Pete,
(my random thoughts) more things to consider

wheels and tyres all in true and weighted correctly, no lumps or bumps on them, how old are the tyres - you could try swapping the rear wheels with each other

checked rear wheel bearings

is the knocking -
constant and/or regular
speed related
still there if you coast out of gear (illegal to do on the road(?) I think)
louder or softer if you steer heavily to either side (thus transferring weight to other side and lifting the other side)
more or less noticeable when braking or accelerating (again shifting weight and level and testing braking noise)

GT or roadster?
Nigel Atkins

Traced such a noise to inside the shock absorber one time.
Paul Walbran

there's a thread elsewhere on the BBS on that very subject
Nigel Atkins

I've had a rattle for years that I could never track down, from behind the passenger seat which was difficult for me to hear anyway, less so for the navigator. While recovering the seats and manouvering the frames on the bench I suddenly heard a very similar sound, and that turned out to be the recliner return spring which was rattling inside its tube. Stuffed that with some hessian, and time will tell, but the point I'm making is that it could be absolutely anything.
Paul Hunt

I have a feeling that your shockers are shot. Are they tube type? Disconnect them then take the car for a gentle run and see if the noise has gone. Take care when driving with the shocks disconnected as the car will be very bouncy. I suspect that you have Spax fear shocks which are notorious for this fault
Iain MacKintosh

Re-con Armstrongs are sited as being noisy as Nigel and Paul mention.
Spax valves are incredibly noisy from new.
Also the the front spring bracket holes are often elliptical, hence the bolt moves to and fro, tapping on the bracket. Try large, heavy washers on the eye bolt and torque up.
Allan Reeling

I had a rattle in the rear one time that turned out to be the boot lid support.
Off subject, but we had a new XJ6 in 1980 that a customer brought back several times for a rattle in the rear. After several inspections, a soft drink can was found in the rear door. We guessed it was left there by an unhappy JRT employee. I still have the UK produced can somewhere.
Cleve Crews

My soft knock came from the metal brake line banging against the bottom of the boot. Bent it out a touch and fixed it. Took me months to find it.
Michael King

Thanks very much, chaps. That's given me a few ideas to look at over the weekend. It's still got its lever arm shocks. I'll give them a good check. They're not leaking, and they seem to damp, but that doesn't stop them having something loose inside. It sounds a heavier sound than a bracket rattling, but I've got to think laterally on this one.

Thanks again. I'll report back!

Pete
P A Allen

Pete. Wire wheels? When the spinner is over tightened, the center of the hub can deform. After that, the spinner can be tightened, but it tightens against the end of the threads on the axle and the wheel can still be not firmly attached to the axle. If you have wire wheels, put the rear up on jack stands, put on the hand brake, and see if you can get movement of the wheel on either side.

Les
Les Bengtson

I know what you mean, Les. My last one had wires. However, this one's got plain old Rostyles (until I've tidied up the set of Revolutions that need to go back on).

Thanks, Pete
P A Allen

Battery not secure? Or possibly the wooden/ press board support/base in the the battery carrier is distorted from years of load bearing allowing the battery to "bounce" slightly?
Hard to see without removing the battery. Cheers

Ray
Ray Bester

A mate had an MGC with a rattle in the back that took ages to find , Ended up being the handbrake cable rattling on something(spring I think)back there
willy
William Revit

Could be that one of the rear shock absorber bolts has come loose. I've had this before and once I'd re-torqued them it was all quite again. In fact I need to check them again as the clunk is back - I suspect the spring washers are no longer springy enough...

Dave Smith GT

Dave,
IIRC there was some talk that spring washer aren't effective even from new so perhaps a nyloc nut instead of plain nut and spring washer combination
Nigel Atkins

Closure at last...........to add to the sum of human knowledge...........

I'd taken on board all your suggestions, but was still at a loss. I even put all new polybushes in rear suspension. This was cheesing me off, as it alternates as a daily driver with an even older Riley.

Anyway, took it for the MoT today and on the front suspension check it showed the front nearside shocker loose on its mounting. Strange, I thought, as I'd checked all nuts and bolts around car prior to test. I asked if I could tighten them up there and then, but they appeared tight, so had to take car away. It transpired bolts were seized in a loosened state if that makes sense, hence not picking it up before. Bolts lubricated and refitted.

No more rattle!

Odd that I thought sound coming from rear of car. My small daughter thought it was coming from under floor on her side, so it must be the way sound carries.

Thanks for all your help.

Pete
P A Allen

This thread was discussed between 16/02/2015 and 20/06/2015

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