Why does my Audi A5 engine make a rattling noise when starting up?

I recently noticed a strange rattling noise coming from my Audi A5's engine when I start it up. It goes away after a few seconds but it's quite alarming. Does anyone know the possible causes and how to fix it?

The most common cause of a rattling noise during startup in an Audi A5 is a worn timing chain tensioner. I'd recommend getting it checked and replaced if necessary by a certified mechanic.