I have a "clank" or rattle from somewhere under or beside the drivers seat in my 4Runner. It occurs when I drive over washboard or sometimes even over manhole cover. I don't think its the exhaust. Any ideas?

I thought it might be the T case counterweight so I removed that and went for a drive. The clank was still there. What does this counterweight do? Reduce drive line vibrations? It weighs about 5lbs and the weight is suspended by rubber to a bracket that attaches to the driver's side of the T case.