My car started to chug on the way to work this morning. I tried it again at lunch and while stationary with the engine running it kept chugging, very lightly but still very noticeable, the whole car seemed to be vibrating. I brought it for a spin and I heard 2 small bangs from the back of the car. I haven't a clue about cars but I'm thinking it could be the exhust. When I parked it up and left the engine running again to go take a look at the exhust the chugging had stopped?? Any ideas what it could be?