Clunk noise in rear of 912

[Geoff_M]

I have a 1965 912 with 5 speed transmission.
When I take off in first gear there is a single mild clunk noise when the torque comes on. If I pull away very gently at minimum revs it doesn't happen. All the engine mounts and transmission mounts are new. I have checked to see if the transmission is hitting the body where it goes through the body to the shift linkage but no signs of that. The CV joints were recently inspected and repacked with grease. The transmission is a reconditioned unit with about 5000 km use. I'm planning a 3000 km trip to TAS in two weeks and don't want serious problems to develop while away.
Any ideas on what might be the cause?

[Merv]

Throw out bearing or release fork Geoff? Also check the clutch cable isn't sticking anywhere, even near the pedal base area. Or muffler now hitting some where with those new engine mounts?

[mrar]

Hi Geoff,
My 66 also began doing this after a long period off the road sitting in an engine out position (maybe the rear bushes didn't like sitting in the raised position). I have had a poke around too to see what is making the noise with little luck. I did install some nos bilstein sport rear shocks in place of the original koni's.Over the weekend I'll release the lower mounts and see if the noise goes away and report back.

[PCM911T]

Alternative thought as you are obviously checking for anything that is loose - fuel filter tapping something/ or if the shocks have been extended for a while ,spray some silicon spray up into the shock covers (gone dry?) and bounce them up and down to lube the inner shafts.
Paul M

[Merv]

Are they new Koni shocks? If so, at times the white poly washers can make some noise

[911ex2]

Engine or trans mount ?

[SM911]

If your transmission mount bolts are slightly too long they will come up and knock the body. I have experienced the same symptoms before. Put a dob of grease on the tip of the bolt, and then take it for a drive, replicate the clunk and check the bolt again.

[Geoff_M]

Thanks everyone for your suggestions.
Progress so far:
Checked the mounts and a bolt on one of the rear rubber blocks was loose. Tightened it.
Plenty of clearance around the tailpipe. No signs of contact.
Checked clutch ajustment.
Removed rear Koni shocks and checked. Both in good condition.
Did another test drive. No change.
Further testing done:
Sound doesn't occur taking off in reverse.
After taking off in first sound heard then back off the speed in first and rapidly accelerate again - no sound.
Rolling along in 2nd 10-15 mph accelerate - hard to detect if it is happening or not due engine and road noise - it isn't a loud noise - thought I might have heard it once.

Just speculating, it seems like there might be some slack somewhere in the drive train which takes up on first moving off and then doesn't slacken off until the next stop. I was hoping for a simple cuase that wouldn't involve taking the engine out!

[PCM911T]

Geoff, I reckon that you should stop worrying and just drive the car sensibly. These are old technology cars and need some time to bed in new components. A long trip will do it a lot of good and by the time you get to the return trip the sound might have disappeared; just have some roadside assistance cover as a back up. If you have any trouble in Tasmania jump onto Porsche Forum Australia and ask for help. There are plenty of Porsche people in Tasmania that would help you out.
Paul M

[Geoff_M]

Thanks Paul,
very comforting words which make sense and give me some confidence to just go !

[Merv]

Geoff since there is no clutch slip or shudder that you have mentioned and no squeal with the clutch in, I suspect that is all OK. If you have checked the shocks' mountings and operation, then they should be OK. Have the spring plate (usually rubber) bushings on the rear torsion bars been replaced? They can make a clunk noise on take off if ovalised and worn. Also, not a job that you have to do immediately. You might also notice some noise over speed bumps.

[Cairns888]

Jack up the back and whack some stands under the torsion bars. Drop the clutch in first gear at idle and see if it’s makes a clunk. Also try it with the hand brake on 1/4 and then 1/2 way. It’s good to have your best mate having a listen underneath with his hand on the gearbox. Obviously have a jack underneath the engine as well..... and cold beer.

[Geoff_M]

Thanks Jim and Merv.

The torsion bar bushings haven't been looked at during my 6 years of ownership and could be very old.
I have a hoist so can do Jim's procedure easily which should be interesting. I'll put the wife in the car and I'll check underneath. I don't think she would be interested in reversing the roles!

[Merv]

Worth checking Geoff. The rubber bushings get hard, worn and out of shape. I found that the new ones I got from Pelican were pretty poor quality and then spent the extra for the Elephant racing (rubber) ones (rear and front). All noises disappeared. I did a thread on the process a couple of years ago. Pull off the torsion bar covers (if there seems to be a noise) and have a look at them. First, measure the angles of the spring plates when the car is at rest and then again at full droop (sic) on the hoist and then set them to the level/angle you want at the end, with adjustment of the splines. Easy really. Good to see the WHS aspects you are planning!

[Geoff_M]

Thanks everyone for your contributions which I highly appreciate.
The noise disappeared last night while the car was sitting in the shed ! I went for a drive today and as hard as I tried I could not get a single noise out of the car. I have done so many things it is now hard to determine what it was. All the investigative work was valuable because I found a few things needing attention during the process.
I have done the Tassie trip twice in the last two years and the car has been very reliable doing 4000 km in two weeks without a problem. I participate in the Classic Rallye which is for 30+ year old cars of any make and follows the Targa Tasmania competition around. We drive on the closed Targa sections in convoy, not competing. I can recommend it to anyone wanting to enjoy their classic cars.

Cheers
Geoff