[stackj]

Ernie,

I thought at first you may be getting noise from an avionics fan, but I don't think that would continue when you shut the master off. I think it would spool down rather quickly. It would probably continue with the avionics master off. My blower comes on with the aircraft master switch.

If you have access to a stethoscope you might try holding it to the face of each of your gyros to see if that helps you define the source.