Tapping Noise??

[Way up North A4]

I have an 1999 A4 1.8TQ with the tiptronic tansmission. The car has 80k miles and is not beaten. Recently at random i get a metallic tapping noise from the engine bay. The car never loses power or idles irratically. It has perfect vacuum and boost. The sound seem to be originating from the back of the block possibly tranny area. The transmission shift smoothly and goes from auto to manual fine. i have no check engine light.

Now in my time as a honda head it is the same sound as if there was a dead spark plug and the piston was hitting that. It can't be that or a valve i don't think cause the car is running perfectly. I do not have access to a vagcom. Any ideas guys?