Strange sound coming from my TT 225
 

I have a year 2000 Audi TT 225 and yesterday I noticed an unusual ticking (almost like a clock!) sound coming from the engine (in the background) - see link to youtube ( ). It can only be heard with the windows open or from outside. The car has 118,000 miles and the cambelt etc was changed at 90K.

Does anyone have any idea what the sound could be?

Thanks in advance!