whatever it may be, there is little to nothing one can do to fix such AIO issues, besides changing a rusty fan if it was the case. Checking or swapping headers, some software tricks if it has one, maybe detach it and give it a shake about, wiggle the liquid & bubbles in it while breathing in some hopium gas. Otherwise RMA warranty or check with the producer.
Quite complicated enclosed system with moving parts, only fans are serviceable. Noise may be one thing as long as cooling is unaffected, but once that gets hit then its obvious something gave in. Most times its the propelar getting damaged or sludged in.
On air not much can happen but fan dying and cause is pretty obvious.