It doesn't appear to be any of the fans, it occurs when power is on regardless of load, it stops when I turn off the power.
From what I can tell it is the pump on the Cooler Master MasterLiquid ML240L 240mm RGB Water Cooling Kit
It similar to what these folks are reporting:
Sorry to hear that you're having some issue with this cooler.
You're welcome to bring your desktop into our service department at your earliest convenience. Our service department does not require an appointment. We'd be happy to take a look at this for you!
Common problem with these AIO's from Cooler Master. It seems they have about a 50% failure rate. Mine failed exactly 3 weeks into ownership of the G436 (same cooling unit as yours). I did not want to leave my new computer with the MC service department for an undetermined period of time so I spent the $85 and replaced it with another of the same cooler. So far so good. If this one fails on me I'll try and squeeze a Noctua air cooler in there. Good luck getting it repaired.
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