I recently discovered a loud rattle coming from my engine bay. It's inconsistent but when it does happen it's only at idle. I popped the hood and it seems like it's behind the engine more towards the bottom. I heard it better putting my ear towards the passenger front wheel. The only thing I can think of is a loose exhaust piece? Maybe the pre- cat or heat shield? It sounds like it's metal on metal.

 

Take a prybar or a really long screwdriver and carefully place it against various parts of the motor, without touching anything moving. Place your ear on the other end of the prybar and you should be able to isolate where the sound is coming from.