Chosen Solution
Sounds like a noise capacitor somewhere on the logic board or the smoothing caps in the power supply is failing. I would first try the power supply. Also you may want to get a good surge suppressor or a UPS so your power supply is not shocked by bad power which can degrade it.
You can also try out an external USB Audio Interface to check if the error persists your PCIe Card may not like your motherboard. Otherwise Dan might be right. (Many 5.1 Mac Pro users complain that a (not loud, quite quiet but still irritating) high pitch noise occurs from the Mac Pro and its headphone port when their Firewire Device/Audio Interface is connected. When disconnecting firewire it disappears.)
