Glad I found this. I'm new to AIOs and experiencing a similar noise with a Ryujin II. Like yours, it seems to be working just fine otherwise. Here's what it sounds like when I crank the pump speed all the way up:
https://www.dropbox.com/s/qbrikxs4bzl4lp3/ryujin2.mp4?dl=0
I'm also curious as to...