Sorry for the delay in replying :/ ... I installed it with the radiator in the front cover, I must accept that i had some struggle (don't know if maybe it was because this is my first time installing water cooling) placing the actual pump over the CPU as I felt the tubes a little hard to flex them. Here is a picture for reference https://imgur.com/LXvZLLk
I felt like the tubes where some short and like I alreay said, hard to move them around.
Today, after maybe 2 weeks, I heard the sound again, for about 5-10 sec, and again that was it, no more since then...
Is it really danger/risky to hear it? Water circuit should be completely air free?
I will do the steps you recommend and comeback with results.