It's the conformation tone to tell you that the alarm system has finally set when you leave the premises.
The volume is controlled within Engineer programming, here is a brief explanation
Entry/Exit Tone Volume
This command sets the volume of the Entry/Exit tone from the internal sounder. If you select option 0, the internal sounder emits no tone. Select a value from 1 (quietest) to 9 (loudest) to set the volume (default 5). The internal sounder demonstrates the volume when you enter the digit.
acorn
March 2010