Thank you MIG and Umai Naa!! for your help. Also thank you for reminding me about those VW self-study programs. I have a similar 68 page document from VW of America.

The reason I did not suspect a timing chain issue is that both chains, the main tensioner and the camshaft adjusters were all replaced 45,000 km ago. Also the noise is not there for every cold start. I left the car unused for 3 days over the weekend. When I started it today, there was no noise. However if the car if not used for a week, the noise will be there. I'll look for an opportunity to record it. In the meantime, I suppose I can't eliminate either the chain tensioner or the mini oil tank. I wonder which is easier to replace? I guess the tensioner?