Why does my Audi 80's engine produce a knocking sound?

I recently noticed that my Audi 80's engine has been producing a knocking sound when I accelerate. It doesn't happen all the time, only when I reach a certain speed. What could be the cause of this and how can I fix it?

This could be caused by a few different things, but one possibility is that you have low-quality fuel in your tank. Try filling up with premium or high-octane fuel to see if the knocking sound goes away.