Why does my Audi A4's engine make a knocking noise when accelerating?

I recently noticed that my Audi A4's engine makes a loud knocking sound when I accelerate. It seems to happen more frequently when the engine is cold. What could be causing this and what should I do?

The most common cause of engine knocking in Audi A4s is low fuel octane. Try using a higher octane fuel and see if the knocking goes away. If it does, stick with the higher octane fuel to prevent future knocking.