I own two of these APC Back UPS Pro BX1500M units that were purchased new. Both of these units have always made this very annoying "booo-eeeep!" sound when plugged into the wall. I've owned backup batteries before and I've never heard this before. Everything works fine otherwise.
This sound stops after switching to battery. It also takes a little bit before the unit will start emitting this sound when plugged back into the wall. Performing a reset on the device by following steps to provided by the manufacturer will cause the sound to stop when plugged back into the wall but resume again after a few minutes.
I've recorded the sound and boosted it to be heard at a normal volume. I have to put my ear to the unit to hear it consistently over the HVAC in my apartment but it it clear as day across a room when it's quiet.
I have recently replaced one of these with a similar APC Back UPS PRO BN1500M2 which does not have this issue. I have owned these for about two years and this sound has always been consistent. I worked around it by placing the units in areas there the sound would not be audible.
I can confirm that this is NOT the sound of any of the relays switching to or from the battery.
What is this sound?
See the following link for the recording I mentioned above:
I'd like a confirmation from someone who is electrically inclined and some kind of explanation as to why it's audible. I've been around all kinds of electronics my whole love and this is the first time I've heard this particular sound.
Here is a reference from another APC owner and a response with the possible culprit:
This noise comes from the battery charging circuit, and it's not necessarily abnormal. The newer APC UPS models use a "switching" or pulsed approach to charging the battery, and depending upon the pulse frequency, you might hear a faint hiss, buzz or squeal. (If any of these noises are really loud, that would strongly indicate a malfunction.)