The first thing I would try is to change all of the fluid and the trans filter. If the fluid is broken down and old it will cause the converter clutch to chatter or clunk when applied. Self test the vehicle for any trans related codes. If it is still doing it after this you will need to remove it dissasemble it and inspect the clutch packs / bands for a problem.
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Shawn
Ford Senior Master Technician
20 Years Ford experience, 10 Year Ford Senior Master, ASE Master with L1, Ford Diesel Certified