I the strange sound the clicking or the harmonic?
If it’s the clicking, I’d change out the PS soon.
If it’s the harmonic, its due to another fan (probably a 140mm near the power supply?) spinning at a rate that causes the 135mm in the supply to vibrate. It’s called a harmonic, it isn’t harmful, per say.
Move any fans that are close to the power supply or get into the bios and disable the MB’s ability to automatically control the fans speed (not the power supplies fan) if it really bugs you.
Sound seems to have stopped now, but thanks for the post