check how you AiO is installed.
as
@ChumP said , remove the clear plastic protecting the part that touches the CPU
also check how the place : better place for the radiator is on the top of the case, exhaust. this way all air is traped on the top, away from the CPU waterblock.
front or rear is ok if tubes are pointed downwards
never install the radiator lower that the waterblock
BEST :
radiator on he top, air stay there.
your AiO should last a long time ad stay quiet and your CPU cool (well, for a 9900k)
OK -ish:
waterblock is slighty lower than the highest point of the radiator, tubes upwards
performance should be ok, but it's possible the pump is noiser than with radiator on the top. that's because some bubbles of air will be trapped in the pump.
should be better with tubes down if possible.
BAD :
waterblock is the highest part of the loop, so air get traped inside.
Very noisy and the CPU get hot
EVIL :
waterblock is filled with air and occasionnaly push some water through.
noise, vibrations, 100C CPU agonizing.
Source :
View: https://www.youtube.com/watch?v=BbGomv195sk