AW2723DF, G2724D, popping sound

I've got strange problem with Alienware 27" AW2723DF. When I leave pc for about half an hour or more and return, some strange popping noise occurs for about 8-10 mins in 2 minute period, and every second sound is less hearable untill its gone. What is your opinion? Don't know if it is normal heating/cooling electronic sound? So far what I tried, changing power cable, bringing extension cable from another room just to make sure its not on same line. But most stange thing is I prepared Alienware monitor to bring to service and bought another Dell G2724D so I have something to work with and guess what? The same thing is happening with brand new G2724D. Just to mention room temperature is around 25-27C (77 to 80.6F) Really dont know what to do any more? Dont know if it is normal or not? Also to add popping sound is happening only on DisplayPort cable. No popping sound using HDMI cable. Thanks!

I recorded a video of popping sound, you can hear it at 1 second, 1 minute 55 second and 4 minute 8 second.

https://www.youtube.com/watch?v=WiNwN1NWWzM