Ford Repair: Thuds from engine, automatic transmition, mustang v6

Question
This problem is on a 2002 Ford Mustang V6 Automatic transmition.  I often hear thud sounds coming from the engine in any gear including park.  Immedately after the thud, the RPMs go up and if in Drive, the car will accelerate slightly, for example i was stopped at a stop light and i heard the thud and the car moved forward a few inches while the brake was applied.  After the thud or clunk sound, the engine revs a little but momentarily returns to normal.  What is the problem? About how much does it cost to have fixed? What problems (if any) may arise if I do not get this serviced?

Answer
Hello and thanks for the question...I'll try to help.
 I couldn't venture a guess as to what the problem is. I would, however, suggest that you bring it back to the dealership and or a transmission shop and have them look it over. It would seem that there is an internal problem with the transmission OR the brake system.
Hope this helps,
Erik