So I know the ZenBook does not have an HDD, but there is a quiet sound that sounds like one coming from the inside of the laptop. The sound grows louder based on the power mode the laptop is on, power saver is the quietest, getting louder at each stage until at maximum performance which is the loudest.
Sounds like the cpu cooling fan.
It is normal for the fan to spin up and work harder under load.
Check to make certain that your cooling opening and exhaust is not clogged with dust.
I would agree with that, but for this circumstance, I am monitoring the CPU load/fan speed while changing power modes, the fan speed stays the same, with load staying the same. It's intermittent and sounds just like a HDD.