1997 Ford Explorer Loud rattling

I've got a loud rattling noise from the engine compartment. The loudest spot I could hear with a stethoscope was from the driver side exhaust manifold. It looks secure though. I don't see any movement from it.

The noise sounds like it's coming from the center or right side of the engine. The noise is also loud underneath, but everything is loud under there when it is running.

I wondered if it could be a lifter, spring, etc. But when listening through the tappet cover it doesn't sound all that loud.

Also, this only occurs at a low or rough idle and quits if I rev up the engine. This leads me to think the noise is external and is something loose that is rattling due to the irratic idle of the engine. What is your idea on this? Could a bolt be caught somewhere and cause this noise?

A good example of the noise would be like someone is tapping on the external side of the exhaust extremely fast or like someone is shaking a few pennies inside of an aluminum can. It is a high pitch rattle like something is hitting thin metal. I was wondering if a bolt got trapped behind the driver side manifold that I don't know about, but space is so tight I just don't know how I'll ever find that out. I also had someone else (not an certified mechanic) listen to the sound just to given an idea of what it sounded like and he said he thought he could hear an exhaust sound with the rattle at times, like a "poof" or passing of exhaust gas that triggered the rattle. I couldn't say much on this because I find the rattle so loud I can't hear anything else.

I know this description and the details I've given aren't a whole lot of help, but if anyone has any idea what this might sound like I would sure appreciate your opinion on the matter.