I reckon it's a bad fan. Used to always happen on old PSUs that didn't use the greatest quality fans. What happens is that the fan rotates off-axis -- it sort of "wobbles" and this creates a lot of noise. It can sometimes be "knocked" back into the correct axis and the sound goes away. Kinda what it sounds like is happening to your PSU fan.
Not something you would expect from a Corsair PSU really but I guess some people are simply unlucky
It could be the fan bearing getting loose. You should check to make sure your fan is running fine (blowing air as it should/cooling), so that the PSU doesn't overheat. Unless you're having any trouble while turning your PC on, having resets while running apps you should be fine. You could also check the voltages to make sure PSU is not damaged.
I had that same issue with one of my old case fans (it was very loud when the PC turns on), but once i press the middle of the fan (the bearing) it was fine (you shouldn't do this btw., especially around PSU). Why I told you to make sure fan is blowing the air is because my case fan wasn't when it had that noise.