Why is my Ford Mondeo engine making a rattling noise?

I recently purchased a Ford Mondeo and have noticed a rattling noise coming from the engine. It only happens when the car is idling and goes away when I accelerate. What could be causing this and should I be worried?

It's possible that the rattling noise is due to a loose heat shield on the exhaust system. This can be easily fixed by tightening the bolts or replacing the heat shield.

Another potential cause could be a faulty serpentine belt or tensioner. This can cause the engine to make strange noises and should be checked by a mechanic.