Narrative:

We received a call from the flight attendant telling us that some passengers were concerned about a noise coming from the floor on the left side of the aircraft just forward of the emergency exit and just behind the leading edge of the left wing. The flight attendant informed us that she had heard anything like this before. The first officer and I; as well as the jumpseater; began to check for any abnormal indications. All systems appeared to be normal. I decided to go into the cabin and check the noise out for myself. As I exited the cockpit and started to walk back to the area of concern; I could hear the noise almost immediately; also the floor over the location had some vibration. I; myself; had never heard anything like it before. I returned to the cockpit and informed the first officer and the jumpseater of the situation. I told them we would have to divert and a major station would be the best place. I also informed the flight attendant of the same. We contacted dispatch; they concurred. We also informed dispatch of our overweight landing. I then told the passengers and flight attendant of our plan. We received an ACARS message from dispatch telling us that we would have to declare an emergency because of the overweight landing. I did declare an emergency. We landed without incident and had a normal taxi to the gate. The only thing I had a problem with was dispatch telling me I had to declare an emergency. The first officer and I questioned it. However; I decided to err on the side of caution; because of the abnormal problem with the aircraft and the overweight landing.

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Original NASA ASRS Text

Title: A B737-300 Captain diverted because of a very loud; unusual noise apparently coming from beneath the floor; left side just after of the leading edge; forward of the emergency exit.

Narrative: We received a call from the Flight Attendant telling us that some passengers were concerned about a noise coming from the floor on the left side of the aircraft just forward of the emergency exit and just behind the leading edge of the left wing. The Flight Attendant informed us that she had heard anything like this before. The First Officer and I; as well as the jumpseater; began to check for any abnormal indications. All systems appeared to be normal. I decided to go into the cabin and check the noise out for myself. As I exited the cockpit and started to walk back to the area of concern; I could hear the noise almost immediately; also the floor over the location had some vibration. I; myself; had never heard anything like it before. I returned to the cockpit and informed the First Officer and the Jumpseater of the situation. I told them we would have to divert and a major station would be the best place. I also informed the Flight Attendant of the same. We contacted Dispatch; they concurred. We also informed Dispatch of our overweight landing. I then told the passengers and Flight Attendant of our plan. We received an ACARS message from Dispatch telling us that we would have to declare an emergency because of the overweight landing. I did declare an emergency. We landed without incident and had a normal taxi to the gate. The only thing I had a problem with was Dispatch telling me I had to declare an emergency. The First Officer and I questioned it. However; I decided to err on the side of caution; because of the abnormal problem with the aircraft and the overweight landing.

Data retrieved from NASA's ASRS site as of July 2013 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.