I am using a old radio shack multimeter that has a frequency setting(cat. no. 22-168A). Checking other plugs that are on a different circuit it shows around the 60Hz. With the audio system it is a hum that almost sounds like a ground loop. When the pc is shutdown the high frequency goes away. Is is possible that it is interference between the computer and other electronics. On this circuit there is my computer, tv, audio system, router, modem, and a external hdd.