I was on an A320, asleep, around the mid-point of the flight at about 30,000ft. I suddenly heard a constant noise like someone hitting the plane from outside with a hammer. The noise wasn't too loud but it was distinct. I looked around, none of the passengers seemed perturbed.
The noise continued for 10-15 minutes, stopped, then started again after another 30 minutes. I was about to bring it to the attention of the crew, but the plane had started its descent by then.
Does anyone know what the noise could have been? And should I have alerted the crew?
Edit: Checked the linked questions. Its not a PTU, because the noises were heard mid-flight not during take-off or landing. Also, the second part of the question, can be answered.