Back-UPS ES 750 buzz/hum noise?

[mmiller7_apc]

This was originally posted on APC forums on 11/9/2011

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I have a Back-UPS ES 750 and it's making an annoying buzz/whistle noise. It's not all the time but when it's there I can hear it loud enough that I can't sleep. My parents have an identical unit that makes zero noise. It's louder when charging (e.g. after a power outage or self test) and it's not the same whistle that happens when it's running on battery power -- this is when it's on AC Mains power. Aside from the annoying sound it seems to work fine and switches between battery and AC power normally.

Here's what I have connected:
-Laptop with 160W power supply
-Laptop with 90W power supply
-16" TV used as external monitor
-USB hub
-Radio charger
-Powered computer speakers
-Coax/Ethernet bridge
-WiFi print server
-Inkjet printer HP PhotoSmart 7260v

PowerChute says I'm pulling between 40 and 200 watts depending on whether it's running/charging my laptops or not. I've checked with my Kill-A-Watt meter and even if I add on the light, clock-radio, weather-radio, and desk lamp that are on the non-backup plugs I'm still only using about 300 Watts peak so it shouldn't be overloaded. I turned it off overnight and it's quiet now in the morning which seems odd as well. I thought it needed more air or something so I propped it up off the carpet using a large ceremic floor tile but that didn't make a difference.

Before someone complains about the printer, our other Back-UPS ES 750 we have has more connected (including 3 network adapters, a cable box, a mid-range desktop tower, inkjet all-in-one, and a second inkjet printer) and it never makes any noise and chugs along even during outages without a hickup.

Has anyone else had this problem with humming or whistling?

[voidstar_apc · Accepted solution]

This was originally posted on APC forums on 11/9/2011

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You could try swapping your parent's BackUPS with your own and see if the noise follows the BackUPS. Sometimes electrical distortions on the power line causes the transformers in devices like the BackUPS to vibrate generating a noise. One person here mentioned tracing a similar issue back to a dimmer switch.