Wambo,
Just an idea but i've had a tapping noise on idle, found mostly when im low on petrol, i had it diagnosed as the charcoal canister and valve today. It also has turned on my ECU light and ruined my fuel economy. I Haven't heard the tapping as much recently but Seat in Durham said that the cam follower replacement common on these forums could be the source.
However i do believe it to be the canister, and its more audible inside the cabin for some reason.