That is the middle marker for the ILS approach. It's a beacon signal that shows you that you are on the correct location for the approach and you can cross check your altitude. There is an OM = outer marker, MM = middle marker and IM = inner marker but most airports only have the MM installed, if any.
This is the answer of my brother-in-law-favorite-all-Airbus-pilot:
On ILS, sometimes there are transmitters that allow you to check altitude points at the good place (OM, MM, IM). There should be sound signal in the cockpit.
