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