Just installed H100i v2 into pc and now when I play games like PUBG or AC Origins I can hear a buzzing sound coming from PC. I checked all fans and put the pump to max to see if it was that but still no luck. It goes away right away when I turn off the game. Any ideas?
One simple test is to turn on v-sync or change your graphics settings. The sound is often associated with the FPS the GPU is producing.
One method for tracking down the exact part is to take a wooden pencil and hold it up to the GPU and your ear. Probe around until you hear the vibrating part. A little glob of silicon caulk (or other non-conductive goo that doesn't fully harden) can help with this.
I suggest starting with some Youtube videos showing coil whine so you can compare it to your noise.
Power supplies can also suffer from coil whine. (As well as motherboard VRMs, but this is quite rare I believe)