When you start the flight. Change the  frequency of NAV1, NAV2, ADF1, ADF2. Change one at a time to see which one of them is triggering it.

Also. Make sure that that the audio for all the navigation aids is turned off. Load your default flight and check to make sure that all the audio lights on the Radio Panel are OFF (excluding Comm1 or Comm2). Re-save the flight if you have turned the audio off on any of the devices.

Mark