I have just had my cambelt tensioner replaced on my turbo(MY00)it was causing a knocking noise when it was cold. Apparently some cars have been fitted faulty tensioners that were not filled with enough oil during manufacture. Dealers should be aware of the problem as there was a bulletin about it. I hope this helps some people with knocking noises.

 

I also had the same problem whith my MY98.

My first thought was...pistonslap again...