So, I unplugged everything from the receiver. Now, all that is plugged in, is the speakers. The hum is still there.

Could it be the surge protector causing this?

Volume levels don't seem to matter, as in, no matter how high or low the volume, I can hear the high pitched hum. Maybe I should try and unplug things from the surge protector? That might/might not help, but that might help? I will try, and report back.