My AIO has been iffy for a while now, i hit good temps and all that But the sound my Kraken x62 makes at high RPM although quiet, its still concerning when i hear an occasional hum after it hits a high RPM. Is this normal? I RMA'd my previous Kraken and is had a similar issue yet was more of a broken pump, im just not sure if this is the same. I am still yet to change the TIM on it so maybe? I dunno at this point.
Faint humming of the pump is normal. I would question why you said 'high RPM' vs. 'just all the time' but I am assuming you have the pump set to PWM cycle. Not my cup of tea as there's really no benefit to PWM controlling a liquid cooling pump - the real 'control' of load temps is where the duty cycle takes place and that is the PWM fans on the radiator, but most people do not realize this.
For example, the humming of a pump should be very, very faint, likely inaudible over case fans and likely even lower than the spinning of an active spindle hard drive.