Loud fans

Hey everyone, so I have an imac 27” late 2015 and I started having this problem a couple of weeks ago! The fans will become so loud ONLY if I watch youtube videos. I watched tons of hours of high quality videos on Netflix, worked on large files in Logic pro and the fans are quiet. Also when a video is playing on youtube but I have any other window open and dont watch the video the fans will stop being loud! Please help