DIAGNOSIS This is the normal end-of-cycle signal, not a malfunction. It is a user-adjustable setting for audible alerts.
FIX To disable it, press and hold the 'Sound' button on the control panel for 3 seconds. The display will show 'Snd 0' or a similar indicator. You can also press 'Sound' to cycle through volume levels (1, 2, 3, 0). Set it to '0' for silent operation.
IF THAT DIDN'T WORK Ensure you are not pressing the 'Start/+30 sec' button after cooking, as this initiates a new 30-second cycle which will also beep when done. Simply open the door to stop all alerts.
HEADS UP The 'Sound' setting also controls button-press tones. Setting it to '0' silences all operational beeps, not just the end signal.