Is it the disc drive or the fan? Can you tell?
Since there's no disc in the drive, it's definitely the fan.
These systems DEMAND reasonably decent airflow. With my media console closed (vented door), the system gets much louder in AC Origins. And at one point it was downright NOISY for just a second. I have a feeling the temperature threshold was passed at that point.
With the door open to my media console the system is quieter during AC Origins. But the fan absolutely changes speed when you play AC Origins versus right after you close the game.
This is quite different to Xbox One S which I don't ever recall changing fan speed audibly.