OK, so just thought I would share a little knowledge that's come to light, as it may save someone a little aggro.
I believe I have found the source of my tapping, and it wasn't what was expected.
On my original build back in 2010, you may remember that following the build, a valve dropped and mashed the various internal parts of the engine to bits. These bits that were mashed also included the piston in cylinder 1. Bugger. At the point the engine was built initially, it had all 4 AGU pistons in the waiting for some abuse.
Now, I have NEVER given this a second thought and to be fair had completely forgotten all about it until now.
What happened following this was that piston 1 was extracted, and replaced with an APY piston from the original engine. Again, didn't give this a second thought at the time. Fitted the piston in a rush to get the engine back together again and completed the rebuild.
This time when rebuilding, naturally I used the best 4 pistons out of the available 8 I had. Meaning that there were 7 AGU pisons and an APY piston. The APY piston was actually one of the ones I had elected to use on the new block as it was in good condition, however I noticed something a tad strange when going to fit this piston to the rod.
I knocked the wrist pin down though the rod until it hit the lower circlip on the other side. Then promptly noticed that there was approx a 2 - 3 mm gap between the end of the wrist pin and the upper circlip. This gave the wrist pin enough room to actually move inside the piston from side to side. And the sound it made when bouncing off the retaining circlips was a tap tap tap.
This would also explain why for a VERY long time (over 2 years) it would tap when cold, and then shut up once it warmed up. As the metalwork inside the engine expanded due to heat, it must have been just enough to grip the wrist pin and hold it stationary.
I talked with Tuffty about this on MSN yesterday and he confirms that the AGU wrist pin would be a tiny amount shorter than the APY due to slight differences in the piston size. This would account for the gap.
So, unwittingly, I fitted a piston with a wrist pin that was too small and had the ability to move from side to side inside the small ends...
Not only this, but at the time (first time taking a 1.8T apart back then) I also had no idea that the AGU engine runs 9.5:1 compression, and the APY 9.1:1. So I ended up building a complete and utter piece of crap engine as it turned out... Ended up having an engine running different compression ratios in different cylinders, as well as having a rattly wrist pin.
Well done me. :-/. I'm off to catch the fail bus....