AA 757 FLL TO ORD. First aisle seat in coach facing flight attendant. Off duty captain in the aisle seat next to me. The engine on the left side was making a terrible noise and the plane kept tilting left each time the noise happened. The look on the flight attendants face was fear. She asked the off duty captain if she should call up front and he quickly responded Yes. She called up front and talked to the cockpit for a few minutes. Then the noise stopped and so did the plane dipping left. They never told us what was wrong and we finished the flight with no issues. That 15 minutes is the only time I thought we may crash.