That's right, @jea48 , it was a large toroidal transformer humming. You answered my query and we discussed what might be causing it before I called the electrician. I'm always appreciative of the help you and others provide.
Your explanation regarding isolated ground rods confirms what I thought. There have been many electrical threads where the OP has believed that isolating grounds either in the house or in the ground will reduce noise. I know there must be only one common ground but was looking for the final word on ground rods. Many thanks.