My 2001 auto started to make a noise that I thought was a heat shield being loose. I tried but could not find anything wrong. One day while taking my son on a drive, I heard it seem to get real bad, then bang! A loud bang, sounded like someone shot the truck, but everything still runs fine. The sound is still there, but not as bad. Now a few weeks later it starts to shift in and out of overdrive like it can't decide which gear to go in around 50mph. I changed the fluid, filter and adjusted the bands....still doing it. What do you think?