My housemate's desktop has a noisy power supply

[Kevin Kuehn]

We need to remember anything being called a ground is not necessarily ground at radio frequencies. When listening to AM indoors with no outside antenna connected, a large portion of the signal is really being re-radiated from the wiring inside your home. Long runs of supposedly grounded wire can make a very good indoor antenna. If you don't believe me try connecting the antenna terminal of your favorite AM set to different ground sources and see how the signal varies.