I've been noticing a strange "thunk" sound and slight jerk on what I believe is the transmission downshifting to first. However, it only happens when releasing the gas pedal at low speeds (stop and go city driving) and it doesn't happen every time (I would say about half of the time). I'm only guessing it is a downshift and therefore a transmission problem because it feels and sounds like a gear shift. Maybe it is something else entirely?Thanks in advance for any advice.