Why is my Audi A2's engine making a high-pitched squealing noise?

I recently started hearing a high-pitched squealing noise from my Audi A2's engine. It only happens when the car is turned on and goes away when I rev the engine. What could be causing this and how can I fix it?

It is most likely due to a loose or worn drive belt. Drive belts can become loose or worn over time and cause this type of noise. Have a mechanic inspect and replace the drive belt if necessary.