Why does my Audi A3 make a rattling noise during acceleration?

I've noticed that there is a strange rattle coming from under the hood when I accelerate in my Audi A3. It seems to be getting worse over time. Can someone please explain why this is happening and how I can fix it?

It is most likely due to a loose heat shield on the exhaust system. These can become loose and start to rattle as the car accelerates. The best way to fix this is to tighten the bolts or replace the heat shield completely.