Okay I put the solution here, because I didn't find any solution while trying to google this on internet. And it took me weeks to understand what was the problem.

So my LFS (my only game) was beginning to constantly "buzzing", a noise from speakers + twitching in video signal. I found many similar problem threads in internet while googling the answer for it. But didnt find any answer. So far I had bought new power supply and cpu (anyway they were old so I tought let's buy new ones). But the buzzing didn't finish. Some days it didn't buzz at all.

The answer was my smartphone connected to the computer via USB...

Never charge your phone through your computer while you are gaming This means also that you need to use your 3G/4G internet connection through WiFi, not USB cable.

edit: looks like its not just the cable, but the phone NEAR my pc. So I need to bring my phone some 3 meters away from my pc for the problem to fully disapear.

Must be hardware or other software issue - just tried charging phone over my laptop,did not notice any problems. Also my 3G modem is connected to other port,also no issues here. Actually direct (wired) connection is (generally) always better than wireless one because copper is better conductor than air. And yes,I know it's technically wrong to say so.

Yes this is very interesting one. I just edited my post and added that its not enough to un-cable the phone, but also keep it far enough from PC (just proved it 5min ago).

Anyway, hardware problem or not, I guess that people encountering this problem, will get the solution by moving the phone in the other end of the room.

The buzzing didn't go directly to my spekers, but to my PC. As I wrote in my first post - "+ twitching in video signal" (and the twitching was synced with the buzzing) or...could it go into display cable and speaker cables in sync, lol i don't know.