This happens e.g. when you scroll in the Chrome browser (or usually very often if you just move the mouse). It’s certainly related to the processor in some way; there’s a direct, noticeable correlation between the loudness/frequency/intermittency of the noise and CPU activity. It’s slightly better when you disconnect AC power, but still noticeable and, if you work long hours, annoying. When compared to MacBook Pro it (which is silen), was very noisy.
I found solution for myself, which is to disable CPU power management. My Lenovo P51 works silently now. There is a better description here: