Whoa, I just noticed something. I have a line conditioner on this outlet too. It clicks and via LED lights, it'll show if it is regulating for a voltage boost or drop. The 7500 was exhibiting the noise more prevalently when the line conditioner showed that there was at the given time a voltage boost which the conditioner was compensating for. When it dropped back down, the hum also dropped.
Looks like I have some troubleshooting/testing to do. I'll try a few different configurations to see if I can notice any patterns.
The problem is I might not always be able to make changes and test since these voltage changes don't always happen in my apartment. Sadly though, they do often.