DHC8 Captain experiences a loud noise coming from the cabin during initial descent. The Flight Attendant informs that the forward Type I emergency exit suddenly began making the noise. Although pressurization is normal; passengers are moved away from the exit; an emergency is declared; and flight diverts to a suitable airport.

Narrative

During initial descent; under instructions to cross ABC at 11;000'; the First Officer and I heard a loud noise. It began very abruptly and we suspected it was coming from the cabin. Cabin Pressurization was verified and at no time during the event was cabin pressure abnormal. I called the Flight Attendant and the noise was very evident through the Interphone when she answered. The Flight Attendant reported that the noise seemed to be coming from the forward Type I emergency exit door and that there was also water on the floor around the door. Although we had no warning lights indicating that any of the doors were not secure; I instructed the Flight Attendant to move passengers away from the Type I door and take the rear cabin jumpseat. Further communications with the Flight Attendant as well as Dispatch were handled by the First Officer and I handled communications with ATC as well as flew the airplane. I declared an emergency and initiated a diversion to ZZZ. We descended and received vectors for the ILS. During the diversion; the First Officer verified the new index of the aircraft as we had shifted passengers in the cabin and informed Dispatch of our condition. At about 6000' and still descending for the approach; the loud noise abruptly went away and we executed a normal approach and landing. On the ground after coordination with Dispatch; Maintenance Control; and contract maintenance at ZZZ; we obtained a ferry permit and flew the aircraft unpressurized and empty back to base.