It's definitely a power line sync'd hum. I say only that it's likely a ground loop, not 100%. Even without a ground pin on the power plug, the wall wart's output can be tied to ground. Besides experimenting with a battery to eliminate the hum, you could try a different wall wart. They have various degrees of isolation from the power line. Grounds loops are sometimes horribly difficult to fix. Sometimes the path of least resistance is to use a ground loop isolator, often used in home and car audio systems. Or do what most of us with external mics do: Use a POE splitter to power both the camera and mic. With most of them you also need to use a 2:1 splitter cable to get the power to both the mic and camera. I've got a few Microseven mics set up this way with no hum, and you should already have the 2:1 splitter cable that ships with the mic.