So I tried a test. Outlaw suggested I try various outlets and if possible, outlets on a different line. When I tried, I thought the different line cleaned it up. But, I was using an extension cord and when I tried it on other outlets on the original line, I couldn't hear it then either. So, I used a power strip to different outlets on the original line, and I could hear it. So, essentially somehow this one extension cord is masking the test.
Now with the power strip, I can hear the hum from the 7500, from 2 other outlets on the same line, with or without a cheater plug. I also was able to find a different extension cord that still allowed me to make the 7500 hum. Using that extension cord, going to a different line, the 7500 still buzzes/hum.
So, I guess it isn't really this one line. It is in my apartment at various locations. I wonder if the electrical stuff in this building is just noisy. I know the light flicker a lot. I wish I could find a loner DC filter, just to try it out and see.