Strange noise comes out from the ups after a power cut

The Back-UPS Office unit has five different tones or alarms:

1. Four beeps every 30 seconds (On Battery LED will be illuminated)
This alarm indicates that the Battery Power Supplied outlets are now supplying battery power as opposed to power from the utility. If the UPS determines that the input utility power is unsafe for your computer equipment, the UPS will switch to battery power.
Note: If the power disturbance is brief, you may only hear one or two beeps.

2. Continuous Beeping (On Battery LED will be illuminated)
This alarm indicates that a Low Battery condition has been reached and that the UPS will soon shutdown. Once the UPS transfers to On Battery operation, it can only stay on battery for a limited amount of time. The UPS will sound this alarm about two minutes before the batteries are totally exhausted. Once exhausted, the UPS will shutdown and turn off its output power to your equipment. If you hear this tone you should immediately shutdown your computer to prevent data loss or corruption.

3. Continuous Beeps for 30 seconds (Back UPS OFFICE 250 and 280 ONLY)
This alarm indicates the unit has detected an Overload condition after it has performed a self-test during start-up.

4. Continuous Two-Tone Beep
This alarm indicates a severe Overload was detected while the unit was on-battery operation. This tone will continue to sound until the unit shuts down.

5. Continuous Tone (On Battery LED will be flashing)
This alarm indicates that the battery-powered outlets are overloaded.

6. Continuous Tone (On Line and On Battery LEDs flashing) Back UPS OFFICE 350, 400, and 500 ONLY
This alarm indicates that an internal UPS fault has occurred.