[video included] APC Back-UPS Pro 1500 buzzing noise with rhythmic pause every 4 seconds

[Kryzoid]

I bought a APC Back-UPS Pro 1500 in 2018 and only just had to replace the battery this week. I noticed this after installing the replacement APCRBC124 battery. 

Video: (buzzing starts at 27 seconds): https://www.youtube.com/watch?v=Gbbek8DwBRo

There's an unexplained "click" at 27 seconds with no associated changes in the little screen. And that's exactly when the rhythmic humming noise starts. Sustained humming with a brief pause every ~four seconds. I've adjusted the volume up so it's easier to hear and added text overlay to mark the pauses.

This only happens when the APC is plugged into the wall and the battery is inside. When unplugged from the wall, or when plugged to the wall but without a battery, this does not happen.

Not sure why this is happening. There are no other variables that have changed besides the new battery. Nothing new plugged in. Nonetheless, I've tried multiple outlets in my home and the same issue happens when turning the unit on. Same thing with a bunch of things plugged into the unit. Same thing after running a self-test.

This unit is inside a bedroom and while the sound isn't too noticeable during the day, it is at night when all connected devices are off/suspended. 

Could it be the battery that's defective? I'm not an electrician so I really have no idea. Any advice greatly appreciated!

Edit: Not sure why/how, but the noise stopped without touching anything. So for anyone coming here via Google with the same issue, maybe just wait? ¯\_(ツ)_/¯

[Kryzoid]

Not sure why/how, but the buzzing noise stopped. So if anyone comes here from Google, maybe just try waiting? ¯\_(ツ)_/¯