The Beeping noise on a UPS is an alert to the user and is typically caused by one of the three issues below:
 

Possibility 1) No Mains Power - Four beeps every 30 seconds (and the amber “On Battery” LED illuminates). The UPS has transferred to “On Battery” because the input voltage has been determined to be unsafe for your connected equipment. or the mains power is out. The alarm will go off once the UPS returns to “On Line” Mains powered operation.
 

Possibility 2) Power Supply issue. The power is being supplied but the supply is being interrupted. This is likely if you have an overload or are running multiple items on one circuit. If it has one, the 'Overload' light is probably showing on the front of the UPS. This is probably something for an electrician to look at your mains power supply and sockets.
 

Possibility 3) Battery Issue - Rapid chirping for one minute, every 5 hours (and red “Replace Battery” LED is on). The UPS has recently performed a self-test of the internal battery and the test has failed. Normal Mains Power is being supplied to the UPS but the battery is either flat or it is not connected properly or there is no battery. A Replacement battery can be ordered through the helpdesk

In All Cases the UPS can be shut down by holding in the power button for a few seconds, however the till should be shut down correctly first.