I had that too, and in the end, coolant (or something else toxic-looking) leaked out the end of the hose. I hadn't cleaned the computer's filter for years, and the ventilation was simply overloaded at some point. Regularly take out the filter and clean it, then this won't happen.

The cooler is about 2 years old and has rarely been used

The punch is installed at the lowest point.
The radiator is installed at least 7 cm higher.
The motherboard is installed horizontally at the bottom of the case. The noise comes a few seconds after the PC is switched on.

Do the noises only occur under load or permanently? How have the CPU temperatures changed since purchase?
Is the pump higher or lower than the radiator body?
How old is the cooler?
What kind of noise is audible, a bubbling or rather a mechanical noise (pump)?

Check whether the pump is installed below the radiator (or at least the hoses hang higher), otherwise the pump will run dry and damage to the pump would be possible.
Check whether the hoses, pump, etc. are damp (possibly a leak?) and the AIO has possibly lost liquid --> remove and shake, there shouldn't be too much audible noise.