First make sure the idle speed has not increased.. then, Check the prop and coupling for looseness.. Before running, when everything is nice and cool,(trans in neutral) grab the coupling and rotate by hand quickly one way then the other, only about 1/8 turn .. if anything is loose you'll feel the "thunk" ..A suspect place is where the shaft enters the coupling... also check the shaft support strut for looseness.. You should be able to find where the strut is bolted to the hull, just in front of the rudder post.. (don't try to tighten the nuts without someone holding the bolts from turning outside the boat) or dive under and try to wiggle everything by hand.. If all that stuff is tight then it starts getting interesting.. The "spring plate" inside the bell housing, the part that drives the transmission would be a suspect, the motor mounts, and finally, the transmission itself.. Have you checked/changed the transmission oil , looking for metallic flakes?? Lots of metal or very black oil could mean a bearing or gear in the process of failing..
