I would listen to the system with the case open and easily accessible in order to try and pinpoint where the noise is coming from. You might want to use a cardboard tube (such as from an empty paper towel roll) up to your ear to help determine the source of the noise.
Motherboards also have coils that can whine. A UPS can also buzz or whine.
Thanks for the suggestion. I'll try that next time I get to the end of a paper towel roll. I guess the other test would be to swap in another PSU. I have an X-560 in my HTPC that would slide right in (same modular cabling).
If I determine that it is in fact the PSU, does anyone know whether will Seasonic replace it through RMA? It's a great unit otherwise.