XSPC Rasa 750 rs360 making weird buzzing noise

It definitely sounds like air. There are sometimes a lot of little bubbles that are still trapped in the blocks and the radiator. Usually if you run the pump for a little bit they will form bigger bubbles which you can get out much easier.

Also, you can try tapping and shaking the parts that have the bubbles to get them to move into the reservoir and fill it up.

Yup, air bubbles, bane of many a coolers life hehe
follow Rubix' advice but also,

*assuming its a simple loop,
Rotate the Rig through all degrees to help get the air out of the rad, on its side,back etc
make sure nothing is loose inside or it can fall around whilst you do it,
so just be patient and work through, it'll be fine
Moto