It is possible you got lucky, or it is possible that CM brand labeled another manufacturers PSU for your 390W unit; CM is a tier 3 manufacturer. Seasonic, Corsair, enermax, XFX are all tier 1. As a tier 3 manufacturer, I wouldn't expect flawless operation, but I'd bet the protection circuitry is probably present, so when the unit does go, it is much less likely than some no-name brand of frying your computer, though I'd replace it anyway.
The buzzing could be (high pitched) a bad capacitor, (any pitch)a bad fan bearing, or (low pitched)winding vibrations. If it is intermitant, it's prob winding vibrations, which won't hurt anything as long as the coating doesn't wear off of the windings.