I've had internal PC noise from video cards (swapping fixed it, but I wanted to be gaming, at the time), motherboard, and power supply. My Seasonic S12 (just before they all got 80+ certs and PCI-e connectors) and GA-P35-DS3R gives me some good quiet outputs if I needed it (all the internal cards I have around are absolute crap, but now noise-free!

). However, I mainly go external, due to a history of such noise problems.
It happens. Isolating the DAC is probably the best/cheapest option (
if you can confirm that a given DAC uses a transformer for coaxial SPDIF input, you shouldn't need optical).