Constant Tone intermittently on my Back UPS NS8 600VA BN600G

[BillP]

Hi,

I've had this ups since 2015. Recently (about a month ago) in the dead of the night I heard a constant tone and when I discovered it was from the unit, the light was flashing. I had to power off the device. When I powered it on, it just turned on and continued as normal. Then again 2 weeks ago the same tone came on. I turned off the device, but turning it on brought the tone back. I had to disconnect my computer for the the UPS to stop the noise. After a couple of hours when I plugged my computer back in, there was no issue. I've seen this happen about 4 times now and more and more frequently.

The manual seems to suggest that this tone is associated with a battery being connected incorrectly, but that dosent seem to be the case. What might be happening here.