Scott,

It might be a ground loop, though. A lot of "S-PDIF" interfaces today
don't actually follow the spec and aren't transformer-isolated. But if
it's taking place with your D100 running off batteries so there is no
other ground connection between the two devices, it's not a ground loop.

My D100 was not running off batteries, I had the DC in plugged in. So
if it's the ground loop, do you have any suggestions? Should I plug
my Tascam to a different wall outlet, etc.?

Also, I was wondering why are you able to dismiss the cable as a
possible culprit?

Thanks a lot for your help!!