I've been on several planes in my life and have always wondered what those pods on a wing actually do. Recently I was on a Boeing 737 (I think) and there were 3 pods under each wing. I've also seen pods on top of a wing and wings without pods. Anyway on my last flight I noticed one of the pods under the wing shaking for the whole 4 hour trip. When we landed I asked a steward what those pods were for and he said they were anti shock stabilizers. I wasn't satisfied with that answer and in talking with others I have also heard them called air flap fairings and someone else said they were extra fuel tanks. What are they really and where on the wing can one expect to see them, top or bottom of wing? I'm leaning towards them being fairings, an aerodynamic pod housing a mechanism for the flaps but I'm not sure.
By the way, the steward said the pod that was shaking does it all the time because it's behind an engine. Didn't exactly fill me with confidence. Are these things designed to take a certain amount of shaking? Is it a good idea to put them behind the engine?