Despite XFX having put-out a high quality 550W unit, power supplies do have a finite lifetime, and that lifetime is shortened by operating the PSU at the upper limits of its load handling capability. I don't know how hard you have run it, but it does sound like it's on the decline.
Singing coils (coil whine) will--eventually--short themselves out, because the noise that you are hearing is a result of actual physical movement of the wires in the coil windings. Constant movement like that will abrade the insulation away and allow adjacent windings to short together. Shorted coils usually result in increased ripple voltage, and that is hard on your motherboard and connected devices.
It is up to you....if it's been ridden hard, then you would want to make replacement a priority. If it has seen mild use, it may be fine...noisy, but fine...but it will fail, eventually. (Not necessarily catastrophically, but it certainly won't be meeting design specs, and that's bad for your hardware.)