I have a 1999 F150 4X4 supercab with the 4-spd auto tranny, 47K miles. This only happens once in a while but most likely when I am towing: When I am gradually slowing down from 50 or more, when I am down to 30-35 mph there is a noticable thunk. It also happens to a lesser extent when I am gradually excellerating, real light on the gas. The truck is otherwise perfectly quite and shifts correctly. Maybe that's why it is so noticeable. Does this truck have a "lock up" tourque converter? Because thats exactly what it sounds like.
p.s. I had Ford service the tranny at 45K and it made no difference, and I forgot to mention it to them then.

Thanks for any help,
Steve M.

 

Yes it has a lockup torque converter.