A high end scanner will read the transmissions codes, but it should also be able to show you the status of the solenoids. That might show you one that is switching status quickly. It will also show you pressure. I believe the accumulators are on the passenger side in the 4R70W and I suppose it's possible the piston could be moving rapidly, that would be odd to me though.
My original couple scanners would read the transmission codes on my vehicles. My $1000 scanner does and has saved me many thousands of dollars.
Here's an exploded view showing the accumulators, reverse is piston 7:
I don't see any way to figure this out without a high end scanner or someone with intimate knowledge of the 4R70W might have an idea.