MJMP, how do you fit such a large brain into that ordinary sized head?
Your troubleshooting suggestion made me try something. In order to isolate the problem, as you were trying to do, I simply plugged a cable directly into the rack without the pedal board. NO NOISE!! So, realizing the problem lies in the pedal board, I begin eliminating pedals (and cables) down the chain until.....VOILA! It's the cable from my pedal board to the rack. I just had to resolder the tip wire, and it was fine....but wait, there's still some noise...not as much...I go hunting again
It is my tuner pedal.
Unfortunately, the tuner powers all the other pedals....but I have a tuner in my rack, so I'm covered. I'll use the tuner pedal as a power supply only. Just the same, I take the pedal apart to see if anything is loose inside. Nope, all solder connections look good, all wires are firm. I open the pedal battery compartment and there's a (many years old) battery still connected. I took that out and the rest of the noise is gone!!
It was a combination of a bad cable and an old dead battery in my tuner pedal. All is well with my preamp!!!!