I have the Raymarine
new E Series 125, and get the same intermittent alarm.
As Chala says, the heading input is from a flux gate compass
or other similar transmitting compass.
In my case, I know the compass is OK, as when the alarm is sounded on the MFD, I still see correct heading data on the auto pilot head
Its a pretty new installation
in my case, I double checked all connections and found no problems.
Raymarine's response was that it could be just a case of input overload on the MFD.
In your case, as it has only recently started with this alarm, I would check the compass and it's connections.