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I recently purchased a UPS for a new server build for my home. Since the server sits next to my desk, I figured I'd plug my Macbook and monitor into the UPS as well. That way if the power goes out, I can finish what I was working on without my Mac going to sleep.

However, there's a high pitched noise coming from my Thunderbolt display when connected to the UPS. I tried another monitor as well (an old Dell), and that one also has the same type of humming noise. Once I switch the displays to another outlet (either direct, or to a surge protector, the humming stops on both monitors. If the monitors are connected to the UPS but turned off, there is no humming.

Why would this be happening? My UPS is a CyberPower CP1500PFCLCD, which is a sinewave UPS. From my understanding, the sinewave feature of the UPS should actually be preventing something like this from happening. I'm guessing there is some sort of ground loop issue or power issue from the UPS that's causing the power supplies in my monitors to behave oddly.

It's an easy fix switching my monitors to another outlet/surge protector, but I'm curious why my UPS would cause this issue. I have a bunch of other electronics connected to the UPS (such as my 2 PSU's from my server, studio monitors, and one or two other devices). All of them function correctly and there's no odd noises coming form them or the UPS. But once I connect either of my monitors and turn them on, the high pitched humming noise begins. The UPS usually sits at 20% load, and all the health reports say it's working well.

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What are the input and output line voltages as reported on the front panel? What happens if nothing is connected to the UPS but the monitor? –  sawdust Jul 29 '13 at 2:29
    
Both input and output voltages are at 121v. If only the monitor is connected, the monitor still makes a humming/whining noise. –  Kris Anderson Jul 30 '13 at 7:44

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