Coil whine is an aesthetic issue. Some part (transformer, inductor, magnetic choke) is vibrating a microscopic amount because it's resonating with the electric currents and fields running through the power supply. I don't believe the power supply is going bad. It's just merely making sounds as it gets older, like an old house floorboards creaking, or strange sounds coming from your grandpa.
Though if you have odors coming from the power supply, -thats- when I'd start looking for a new one. I've had a few whiny PSU's (and GPU's) but none of them were lemons.
Intel Core i9-9900k (Silicon Lottery 5.1GHz @ 5.3GHz 1.36 VCore)
ASUS ROG MAXIMUS XI APEX
G.SKILL F4-4000C19D-32GTZSW (2x16GiB 4000MHz 19-19-19-39 2T, tuned to 4133MHz 14-17-17-31 2T)
EVGA NVIDIA GeForce GTX 1080 Ti FTW3
Seasonic PRIME 1300W 80+ Platinum