I was sitting in line waiting at the carwash yesterday when I noticed this odd phenomena. The truck was in park and was idling when I felt this "thud". It was almost like someone tapped me from behind except nobody was there, so I shrugged it off. Well, as I waited in line it continued to happen, but it was not at regular intervals. The sterring wheel was straight and not turned either direction. It almost seemed that I could feel it more through the seat than anywhere else. Anyone else notice anything like this? Thanks for any help.