Interesting... my experience was very similar... a bright flash and 2 or 3 loud bangs. I thought an engine might have exploded (or something more serious), but after a few seconds realized the plane was still flying normally. And I agree, this was one of the few experiences that really had me worried. After we landed, they had a team of UAL people inspecting the outside of the plane and underneath the nose cone. But good to know it's not serious while flying.