Rattling noise under hood

Idler pulley and tensioner problems are not that uncommon but I believe you're going to find the problem is related to the timing chain tensioner or guides.

And easy way to tell is to simply remove the serpentine belt and start the car. If the sound remains it's not anything associated with the belt, idlers, tensioner or accessories. That leaves what's going on behind the timing chain cover as the most likely suspect.