Dell Inspiron 15 7567 ticking sound issue

Hi!

I have bought a Dell Inspiron 15 7567, and yesterday I noticed, that my laptop makes very quiet ticking sound. The sound very similar to HDD's head movement sound, the only problem is, that in my laptop there is no HDD. I use just 2 SSD (1 X M.2 PCI-E x4 [Samsung 960 EVO], and 1 X SATA 3 2.5" [Samsung 850 EVO]). In this model there's no optical drive, so the only mechanical part of this laptop, that coud generate this sound, if it would be mechanical sound, is the fan system. but strangely the ticking sound only hearable when the fans stop spinning.

So I don't think, that the fans would create this sound, because when I can see in AIDA64 that fans speed is 0 RPM, the ticking sound exactly then hearable.

Additionaly strangely, when I unplug the laptop from AC, the ticking sound almost stops (It not stops, but the click sound becomes rare)... After I plug the AC back, the ticking continues...

Do somebody sensed the same? Is it normal? My opinion, that the sound caused by on of the electronical units bad groundings, or something. Although there's no power or other issue with my laptop now, but I don't want to wait until it gets bad...

Thanks!