Depending on the unit, thats either telling you the battery is dead, the power is out, its overloaded (too much or too large equipment) or there's another problem with the unit. If you have the cable that should have come with the unit, you can connect it to a computer and install the software for a diagnosis.
If you unplug or turn off everything connected to it and it still squeals, the battery is probably dead, but it may be that something is wrong with the UPS itself, and you should send it back under warranty, or get a replacement.
Answers & Comments
Depending on the unit, thats either telling you the battery is dead, the power is out, its overloaded (too much or too large equipment) or there's another problem with the unit. If you have the cable that should have come with the unit, you can connect it to a computer and install the software for a diagnosis.
If you unplug or turn off everything connected to it and it still squeals, the battery is probably dead, but it may be that something is wrong with the UPS itself, and you should send it back under warranty, or get a replacement.