Hi Les,
So it seems that with the MAX660 still on the board, except with the Vout pin lifted, and the -5V powered externally, the noise is still not present. So that means that the MAX660 is not injecting noise back into the +5V rail but only onto the -5V rail. With that discovery, I think I should focus on how I can best patch the MAX660 (drive with external crystal/decouple better?) OR change to another -5V regulator with a much higher frequency built-in oscillator.
I think I am going to see if attaching a crystal is a viable option and report back.
Thanks again,
-Bruce