Tinny rattle/high pitched grind noise

[adrianTV12]

Hi,
I have used your website many times in my 5 year ownership of my early UK Series 3 V12 (1971) and very helpful it has been.

So here i am having used your advice for years but only just having 'signed up' which I guess is not that unusual.... So being honest about it, i have a problem i cannot easily identify and would I like to ask the pro's for help.

I have a noise which is becoming more frequent but which started out as something that only happened once the car had been driven for over an hour and only upon acceleration. Now it happens from cold too but still only when accelerating. The noise seems to come from the middle to rear of the car and is best described as someone hitting a tin can with a spanner to a regular beat. Its a really 'tinny' noise. The frequency of the noise changes with road speed, but only on acceleration. It sounds like something is rubbing or tapping on something made of thin metal. There is no noise whilst the car is stationary even when the engine is running & or revved, it only happens when its engaged in gear and moving. If i lift off the throttle there is no noise. There is no noise even doing 80 / 90 mph+ as long as i'm not accelerating reasonably hard. It happens within 5 metres of setting off from standstill and only if I accelerate reasonably briskly. If I set off really slowly barely touching the accelerator than no noise. I thought the culprit was the dust shields that are on the rear drive shafts where they go part way into the rear hubs (protecting the out UJ's). I tapped on those and it made exactly the same sort of noise i am experiencing. I removed those and still the same noise. I thought it might be from the front exhaust heat sheilds (noises like this can travel) but I temporarily packed those with rockwool to silence them and its not them. However you can probably imagine the sort of noise i'm talking about given the things I am looking at. The frequency of the tapping is faster than the frequency of the rear roadwheel wheel revolution, so i guess that removes from the equation hubs, driveshafts and anything attached to those items. That leaves the diff, propshaft and clutch/bell housing (its a manual S3, not auto). I can't see anything where any of those have rubbed though on anything. The rear suspension unit bushes look good. The rear engine mount (attached to the gearbox) was changed 12 months ago and long before the new noise arrived and that looks good too. The clutch release bearing rumbles a little if I over depress the clutch but thats been like that for ages and no worse than before the new tinny noise arrived. So it has me stumped at the moment, but I know someone else will have had the same problem. I suspect its not the diff, there is no notchiness on the diff (by the way its a LSD) and the noise is too tinny. I know the rear plate of the diff is thin steel but there is nothing that can come into contact with that as far as I can see. I clearly hope its not the clutch given the work involved in that, but surely if the clutch was the cause then it would make the noise whether accelerating or not?
So, I have tried to give as full a description as possible and any of your suggestions would be very gratefully received.
Thanks in advance
Adrian

[Hugo]

Couldn't be just a loose baffle in the silencers, could it? Bit of a long shot, but you could try giving the mufflers a wallop & see if they rattle?

[phil.dobson@mac.com]

have you tried this with the rear of the car on axel stands ? (suitably chocked at the front etc etc.) you might be able to get a location on the noise. otherwise you might try a go pro type camera stuck at various points underneath whist you test drive it.

[adrianTV12]

Hi, thanks for the replies. I removed the exhaust system and shaken the thing violently and no sign of anything loose. Good suggestion though, I hadn't considered it.. Similarly, unless there is load
on the rear wheels from being in contact with the ground then there is no noise.

I have almost certainly found the problem though. I have unbolted the rear end of the propshaft where it meets the diff and whilst it looked and felt fine in situ bolted up, once separated from the diff there is too much free play and a metallic click when I turn it and shake it. It must be dry as a bone in there. It's one of the sealed for life type with no grease nipple, and it is clear it's life is over!! One question though please. Can the propshaft uj's (front and rear) be changed with the propshaft in situ or do I need to remove the propshaft (and therefore the whole rear suspension unit) to change the propshaft uj's?
Thanks. Adrian.

[chrisfell]

I think it is a case of dropping the rear suspension cage to extract the prop. Not a difficult job of you are handy with spanners.

If the replacement UJs are also sealed for life, remember that life will be short. I fitted my replacement UJs with grease nipples, which get a shot of grease during every winter service. The replacements had a grub screw where there would have been a grease nipple. Grease nipples are cheap as chips on eBay or your favourite online shop.

[adrianTV12]

Thanks. Yes, I'll certainly fit ones with grease nipples, I was surprised someone had put the sealed ones in there. I've dropped the rear suspension unit before, I was just hoping to be lazy but I guess I should grasp the nettle and and drop it which would make popping the uj's in and out easier anyway.
Thanks again
Adrian