This summer I have been plagued with a new strange rattle.  It has been bothersome at around 40 MPH.  I spend a fair amount of time in that speed range when I'm going to and coming from trail heads.  I could tell that it was a harmonic vibration associated with engine RPM.  It would also show up for a short period of time as I would get close to shifting from 2nd into 3rd.  Today I learned that if I tapped on the floor in front of the rear seat (just about where rear seat passengers would put their feet) I could duplicate the rattle when parked.  It sounded as though there were a plate of metal or an end wrench resting on the floor that vibrated when the floor was tapped.  Since there was nothing in view I asked my wife to tap the floor while I crawled underneath to find whatever it was that had come loose.  I could see nothing, but could still hear it very clearly from underneath.  She is a patient wife and continued to tap while I continued to search.  NOTHING was loose.  Nothing was even attached where the rattle was.  Finally I decided something must be in the forward hat channel running crosswise under the bed--something like a small end wrench.  Then I remembered that wooden pieces had been installed during assembly.  I used a reproduction tub when I restored my Jeep 6 years ago.  It started to make sense.  The strips of wood installed in the Philippines (probably teak wood) were now shrinking due to being in the desert southwest.  This summer especially, we have been very hot and dry.  So I decided to drill a couple of small holes in the bottom of the hat channel but not into the wood.  Then I installed a couple of stainless steel screws to secure the wood to the bottom of the hat channel.  Test drive proved successful: No more harmonic rattle. 
This may not apply to anyone except those who live in a dry climate.  And it may not apply to original tubs.  I don't remember such a rattle on my old CJs when I was a kid, but they had so many rattles I wouldn't have noticed.
Just for what it worth.

Actually I would prefer having no wood anywhere in a vehicle body. It's a sponge that will promote rusting. It was the way for many early cars to have wood frames with metal over it. Ford gave that up long before Chevy did.