300TDI Disco engine woes

Hello All.
Im trying to find and fix a noise on my fathers 1994 300tdi disco and im fast running out of options other than full engine strip.
Having recently failed it's MOT on a few items unfortunately one of them being the rear floorpan the disco got taken off the road and stood for about a week. My dad arranged to have the welding work done while I did the mechanical failure work. Having removed the rear interior to save a bit of labour cost at the welders, we came to start the engine to drive upto the welders.
We had to jump start due to battery problems and left the disco ticking over while putting the booster pack away. This is the first time we noticed the noise and before this point it had ran like a dream.
It was hardly noticable at first but having the bonnet already up probably drew my attention. It sounded like a rattly knock on tickover that went away when I slightly increased the revs, but the longer the engine stood ticking over while we tried to pin point the noise the worse it got.
I thought it maybe the fan belt tensioner as the belt seemed to whacker on tickover and smooth out when the revs were increased but as soon as you let the revs drop again the noise and the belt whacker got slightly worse. So I removed the main fan belt and the aircon belt and started the engine again. The noise was still present and the longer it ran the worse it got. I checked all fliud levels and made sure the oil pressure light was clearing before continuing.
With both auxillary belts removed and noise still present I revved the engine slightly and promtly s##t myself as it sounded like someone had just chucked a bolt inside the engine, as quickly as I could and before something came through the side of the block I switched the engine off.
Fearing the worst I had a good look round and couldnt see any engine parts on the floor, I turned the engine over with a ratchet and everything seemed ok. With one hand on the ignition key and both ears pointed towards the engine bay I cautiosly started the engine, it started first turn and ran smooth as you like. Completly confused I let it idle for a while before attempting to rev it slightly, nothing no sign of the noise. Then with the engine ticking over faultlessly for a while there was an almighty crack and the noise was back again.

There is no pattern to the noise, it doesnt do it all the time, when the noise is present the engine doesnt missfire. I have also removed the front timing case and checked the cambelt and all seems well.

If you have read this far you deserve a medal. Any help or ideas greatly appreciated.

Thank you.

might be worth checking the vacuum pump - they make orrible noises when they're on the way out, but don't affect the running of the injun.

A friend at work suggested same thing, but with not having a spare and nothing to blank the hole in the block with I removed the vacuum pump and figured out how to remove the plunger that is driven off the camshaft. I refitted the pump so I could start the engine but obviously the camshaft is now not driving the pump.
Turned the key and held my breath and for about 30 seconds all was well and a smile statred to appear, then crack and the noise was back again.

I have also removed the starter to make sure it wasnt engaging while the engine was running and none of the ring gear or starter teeth seem damaged. I aslo checked to make sure all the torque converter bolts were still in. If there is only four and I was checking the right bolts then there all still there.
I have also removed the rocker cover and checked the rocker assembly, valve clearances, pushrods and shone a torch down to see if there was anything obviously wrong with the cam followers.

More ideas greatfully accepted. PLEASE lol