Thank you for the alternator idea, I did the old long screwdriver to ear stethoscope trick on the alternator when it was running and no nasty noises but worth double checking. Thanks once again.
Sounds like you've tried all the obvious things I can think about and, being a mechanically minded person you'll have a good ear for the more common noises anyway. I'd been thinking maybe the pulley bearings in the air con compressor, I've heard some of them making some unusual noises, but you've eliminated that.
These sort of noises can be most frustrating to track down. The first car I bought after I got married was a 105E Anglia (base model) - Heluva come down from my Mk1 Dragoon Red 1500 Cortina GT but marriage makes you broke! I got a good deal on it because it had a very slight knock in the engine which I stupidly, with the arrogance of youth, thought I could easily fix. It was a slightly strange and not very loud noise which was loudest at idle and decreased in volume whenever you stood on the gas. I really didn't know what it was but hoped it was valve gear related. Unfortunately couldn't track it down despite stripping the rockers off and looking for worn bushes, rocker arms and shaft, I'd hoped to find worn rocker arm tips. Then I thought it might be something to do with the mechanical fuel lift pump, but no. Anyway the noise got no louder but didn't go away either. Listening to it every day, and the fact it wasn't getting worse quickly, I began to suspect either piston slap or little ends, so eventually I recommissioned one of my old mopeds to ride to work, took the Anglia engine out and stripped it completely. I was quite surprised to find someone had obviously recently been in there before me. Lots of new gaskets etc. To cut a long story short, it had obviously had a cheap "refresh" done on it. The crank had had it's big ends reground 10 thou down with new shells but the mains hadn't been touched, not even the shells. The bores were quite good with only a small ridge at their tops and the piston skirt to bore clearance was almost to new spec. I noticed it had new rings though. So there I was with a pile of components and, although some showed some wear, I really couldn't fault any of them.
At that time I was fresh out of college so my head was full of all the latest "stuff" and I suddenly remembered reading an article about Cords Rings (Piston rings) where they were advertising their re-ring sets. One feature of which was that the top ring featured a "ridge dodger". This was a small step in the outer diameter of the ring so that the top of the ring didn't go quite, by just a few thou, as far up the bore when the piston was at TDC. These rings were particularly intended for fitting to an engine where a rebore was not considered necessary but a small wear ridge is evident at the top of the bore. Hmm? I wondered. So I bought a set and rebuilt the engine with them taking the place of the almost new standard rings which had been in the engine. No more noise! The new standard rings , or one of them anyway, must have been just kissing the ridge at the top of one, or more, of the bores. That engine then ran faultlessly, including several trips London to Edinburgh and back visiting in laws, (It had actually done that journey at least once with the knock and without it getting any louder) It went on for the next 4 years until the car failed it's MOT when I discovered that someone had stuffed the sills with newspapers over which they had glass fibred and filled and sanded down very carefully until you really couldn't see there was actually no metal there! She went to the happy hunting grounds and I bought a Mk1 1600 Triumph Vitesse. Interesting handling with it's swing rear axle setup! I hope someone got the Anglia engine as I'm sure it had many good miles still in it - but who could have guessed it was the rings that were the problem?
Anyone know if cords are still in business? Those happy days are pretty much over now I suppose. I was looking into recon engines for a friend's Seat the other day and came across the fact that you can't dismantle the bottom end on this model because the block warps as the main bearing cradle is slackened and it's impossible to reassemble it - you have to buy a genuine VAG short block - I believe it's not the only one with this problem either? What a crazy world we live in!