Very few cases that internal transmission parts that have damage do not do signs of something to that effect in the oil.

 

Sounds weird I know. I'd have though I'd have seen evidence in the oil, I agree.

Given what I've already completed, and failing the scenario that BOTH drive shafts have gone at the same time, or indeed BOTH wheel bearings are also gone, I can't think what else?

I was open to the idea of the dual mass flywheel, or clutch assembly, but that spins at the same speed as the crankshaft, and the noise isn't in sync with that. The noise is a clunk per one rotation of the transmission output, commensurate to road/wheel speed. I also think the gearbox input shaft spins the same as the crank shaft, so again one assumes the problem must indeed lay between input shaft of box, and differential? Seeing as I can't separate the box from the diff I'm going to chance the whole unit.

Totally open to anyone else's brains on this matter, hence this thread on this very experienced forum

Thanks again for all the advice and supprt!

 

The limits I have is that you are in the GB. I have found that there seems to be less qualified trained and having certification and then at least 5 years' experience prior getting a certification in automotive service shop independently owned? Maybe it is only based on forums I am on but every time I suggest to find one I get the same responses? Brings back memories when was heavily invested in older British vehicles adding line of parts and aftermarket upgrades to them. Most simply had the lack of qualified service mechanics.
The other thing in your ending of the last response, when you ask that in that way you are going to get every possible guessing and web surfers looking to add info. LOL

Simply the lack of material in your oil which in itself is very unusual because even normal wear you would see some partials in the oil, that no one yet has done an audio test to target exactly wear the concern is, and not doing the basic inspections and checking on 2 type of vehcle lifts , You are in for either a lot of guessing or never finding the problem very quickly.

In my world and how I run my service shop I manage is there is no such thing as a problem that cannot be found and there is no such problem that can't be found with myself or one of my ASE Techs in an hour. If they can't find the problem, they get me we talk it over and the problem within 30 minutes after then is found. Not bragging just fact. And most Automotive training institution that have a shop will often challenge the students by a timed test to find a problem that was created by the instructor. I been there and done that a few times in my life.

I wish I could help you more and truly hope you find the problem without too much investment .

 

Conclusion - gearbox was the problem!

The ‘new’ (second hand box from a Mazda breakers yard approx 60K miles) has solved the knocking/clunking noise, and confirmed my diagnosis was correct.

It took 1.5 days and two people to get it done. Battery, air intake, boost hosing, EGR, all needed removing. Engine braced. Clutch related cables and slave cylinders off along with anything mounted etc. Starter motor out. Below we dropped the sub frame out, with struts and hubs all disconnected. This needed doing for clearance.

I hope this small journey as documented on this thread may help others in the future.

A single knock per single rotation, heard through the car body interior, irrespective of clutch in/out and relative to road speed did in my case indicate a gearbox/differential issue.

Precisely what inside my old box is wrong I don’t know. The primary method I used to locate the fault was as detailed above. The jacking up of the front of the car (both wheels), and the holding of each wheel side (one at a time) from engine on/neutral. By climbing through the gears and still holding one side the noise was present. Repeating this on the other side and the noise still being present was indeed an excellent way to remove other considerations including wheel bearings, disc rotors, tyre issues, drive shafts, CV joints and the intermediate bearing.

As my noise wasn’t affected by depressing the clutch, of relative to the crank speed it did indeed prove a valid method of locating the problem

Thanks again to contributors along the way!

Best,

Will

 

I'm glad you resolved the problem. Are you at least going to quickly open the old box and take a look to know what the problem was?

 

your own ear (like an stethoscope) on that stick to pinpoint where the noise is coming from. Let us know what was the problem, good luck