To me it sounds like the fan inside of the pump in the AIO came loose?
Next troubleshooting step. With the PC on, NOT under load, Unplug ONE of the fans, see if it stops. If not, plug it back in and try the other fan. Then i would boot into bios and unplug both fans to see if it really is the fan or not. If you still hear it with the fans unplugged you can test the pump in bios by changing the curve if it allows you too