A B737-800 flight crew was alerted to a noisy stabilizer trim condition by an alert Flight Attendant.

Narrative

During the initial climb the First Flight Attendant called. She said she had heard a banging sound; like someone was banging underneath her jumpseat. I asked if it was banging right now and she said no. Through more questions; I was able to determine that the noise started shortly after takeoff and ended soon after. I explained that the nose gear is right under their jumpseat and that it may have originated from the gear retracting.Later during the climb she called again and the Captain answered. She said she had heard the noise again. The Captain realized he was trimming the airplane at the time. We determined that the noise only occurred using the main electric trim. At FL290 I went to the Flight Attendant jumpseat area to listen for the noise as the Captain was trimming. I heard a muted sound when the Captain was trimming. I went back to the cockpit and the Captain and I discussed our options. We decided to leave the autopilot on as long as possible during the descent and approach thereby minimizing the use of the main electric trim. We landed at our destination without incident.