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.
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Jim Stack
Richmond, VA
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