Why is my Audi A4's engine making a grinding noise?

I've noticed that my Audi A4's engine is making a strange grinding noise. It only happens when I first start the car and then goes away after a few minutes. What could be causing this and how can I fix it?

The grinding noise could be caused by a worn out timing belt or a loose belt tensioner. It's important to get this checked out and replaced if necessary, as a broken timing belt can cause serious engine damage.