That rattle sound? If so, it is fan bearing noise.
The noise is still there even when i stopped the fans from spinning with my finger.
Did you stop all the fans at once? Or one-by-one? Since there can be more than 1 fan that has this bearing rattle.
You said that fans are connected to the hub. Can you control the fans RPM? Since with this, you could eliminate (or lessen) the bearing rattle.
Some fans will develop bearing rattle (instability) at fixed RPM range.
E.g out of the 6x NZXT AER140 RGB fans that i have between two PCs, 3x of them produce bearing rattle when fan is spinning ~1200-1400 RPM. If fan spins slower or faster than that affected range, there is no bearing rattle.
So, increase/decrease your fan RPM and hear at which point the noise either lessens or disappears completely.
In a chance it isn't bearing rattle, then it could be vibration resonance as well. Tighten or loosen the fan screws, to get rid of the vibration noise. There are even noise dampening corners/washers and rubber "screws" that you can use to get rid of fan vibration noise. For the latter, these things,
amazon:
https://www.amazon.com/DKARDU-Reducing-Silicone-Anti-Vibration-Computer/dp/B0C1444V9W
I used to use those rubber screws for some time with my old AMD build, to see how they fared. Difficult to install. Even more difficult to take off. But since my fans didn't produce vibration noise to begin with, they had no use for me. But at least i got the experience and knowledge about them.
