Why is the Audi A4 engine making a rattling noise?

I recently purchased a used Audi A4 and I've noticed a rattling noise coming from the engine. It usually happens when starting the car or when idling. What could be causing this issue and how can it be fixed?

The rattling noise could be due to a worn timing chain or tensioner in the engine. It's important to have this checked and replaced by a professional mechanic as soon as possible to avoid any further damage to the engine.