I have a 2002 Accord LX 4-cyl that has been making a metallic rattling/resonating noise from somewhere under the floor of the car on the passenger side. I inspected underneath the car this weekend and it is not the exhaust, exhaust mounts nor heat shields. But when I simply hit my hand against the floor of the car on the right side, I heard the resonating noise! There are no moving parts or bolts in that area, so what could be causing the noise?

 

Strange, I would have said its the exhaust heat shield.

In addition to the heat shields around the catalyst, there are moreheat shields above the pipe & to the rear, fastened to the floorpan. Have you ruled THOSE out?