98 GC transmission rattle in hot temp

[sportutilityvictim]

Anyone ever experience a rattling/knocking from their tranny under heavy foot driving or when it is extremely hot out? The rattle sounds like a shaking a marble in a plastic cup as fast as you can, or perhaps like a loose flywheel, however it only happens when it is driven hard for extended time or 90 degrees out & it will go away within a few minutes of turning the heat on full blast to reduce the engine temp which seems to allow the radiator to drop the tranny temp a few degrees.

It is quite predictable, the first time it happened was on a trip into the mountains, after an extended climb I stopped for gas and when I restarted I heard a raquet which I thought was a YZ 80 coming around on the side of the building. I went to pull out & realized the noise was following me, stopped took a look and saw nothing unusual so lacking other options, I drove on. The racquet continued sounding like a popsicle stick in a fan for a few easy miles & went away.

The vehicle is well maintained, runs consistantly between 190-200 on the guage (center) except when under load on hot days it will hover near 210 and like clockwork the tranny will start rattling until I turn the heat on & bring the temp to around 200. I thought about a cooler t-stat but since the existing one is wide open already at 200, I don't think it will help.
A tranny cooler is a given, but before I patch the problem, I would like to solve it since it should not need an aftermarket cooler to perform properly.


The only thing I can think of that would make noise like that is a check ball rattling in a valve.

Any ideas or similar experiences?

Thanks for your help/insight
suv