I have an auto 03 GT. It shifts from 1st to 2nd fine and from 2nd to 3rd fine as well. It makes a knocking noise when going from 3rd to 4th. I just changed the transmission fluid and filter expecting it to help but it still does it. What could be the problem?
In the past, I've heard the pump, on the front of an automatic transmission, make a knocking noise.
If you take it to a reputable transmission shop, they can probably tell you what the likely cause of the noise is, and give you an estimate for the repairs.