Much depends on the actual noise being made ( I can only make assumptions that my experiences of the 'sounds like' noises are similar to yours ).
It likely is air and cavitation - the noise made by the pump impeller when air is being introduced into the coolant as it runs. The same thing that causes noises by boat propellers under the water.
Nearly all AIOs have air in them, that's a simple fact we have to account for. They are never filled 100% to capacity without air. If a liquid cooler makes any sort of sloshing sound when you shake it, then it has air. Something that is filled completely with liquid has no ability to 'slosh' and displace air, and therefore would not make noise.
A few things can be attempted - laying the PC down on the side, or tilting it while tapping on the pump unit and the tubing. Also, depending on the tube orientation leaving the pump and the tube orientation on the radiator and how it is mounted in the case, this might take longer or shorter amounts of time.
Long story short - liquid cooling pumps do not like to pump air. They are not good at it and when a complete airlock occurs, temps can spike and continue to rise until the air blockage is removed. Air bubbles, even small ones, can cause noise when the impeller is moving air+coolant and this is likely what you are hearing...sometimes sounds like a garbage disposal when the sink is full of water, or kind of a gurgling/growling sound.