i have a similar problem too idk what it is and right now the tranny is off the car so i cant mess with it now but im interested in what it is and mine only does it sometimes and also backfires really bad when it does. im considering getting a fp gauge when the car is back together and see if its fuel. you should also try to get a fp gauge and see what it says i believe it should be around 25-28 psi but dont quote me on those numbers.