If there was no need for a fill cap on a car radiator why do they have one? Reason is because there is coolant loose same as engines use a little oil between oil changes .. and automotive coolant should be changed /refurbished every couple of years too, same as liquid cooling in computers.
I have Cryorig A80 which has same pump ans A40. Only difference is fan and radiator size. Pump noise is reason I took it out of my system after testing. While it's not very loud at idle it was the loudest part of my systems. Some people don't mind the pump sound, some can't stand it .. I'm one of the latter. Baden, if you want a Cryorig CLC drop me a note. It is really a CLC, sub-group of AIO and pump is not very quiet and only flows about as much coolant and drunk can urinate. If you are not going to be doing serious overclocking CLCs are okay.
I've been tracking CLC users for years and have found a definite correlation between CLC users with high heat overclocked CPUs having problems after about 1.5-2 years of use. That is when they start hearing pump noises like air moving through system and then a month or so later pump fails.