Humming noise in a PC

[Guest]

So, a couple of times a day, this weird sound comes out of my PC. It lasts about 3 - 5 secs. It is quite unpredictable, but I'd say it often comes when I run a game (just a guess). It's hard for me to describe it, but I'd say it's some kind of (not loud) low-pitched humming, electric motor-like (@ low rpm) sound. I wouldn't say it's coil whine.
The thing is, I have no HDDs, just an SSD so I have no guesses about what it could be. If I had an HDD, it would be my first guess. The system is custom built with parts listed below.

i5 6500 (stock cooling)
Asus H170 Pro Gaming
8GB Kingston DDR4
Samsung 850 EVO
Supernova G2 550W
LG ODD
Enthoo Pro (stock cooling)

Thanks!

 

[CountMike]

Could be a dry bearing in fan.

 

[Guest]

Could be a dry bearing in fan.

I tried ramping fans up and down in BIOS. With case fans there was almost no difference. The audible difference was in the CPU cooler, but it just gets louder, it doesn't make the mentioned noise.
BTW, it just happened again, as I was writing this.