on initial start up there is a dull thud when tranny changes from 1st to 2nd and 2nd to 3rd.only does it for first change every time engine is turned off and then restarted.
What year, milage? was the transmission oil ever changed on your vehicule?
Also, check the level of the transmission fluid. The instructions are in the owners manual.
The Tucson has had many transmission issues. Be sure you ask your dealer for help in case you qualify for a recall or technical service bulletin, or the 2016 “fix.” Consumer Reports had the fix applied to their Tucson and had mixed results.