There normally is only 2 things that will "clunk" back there. The first is the ujoints and the 2nd is the gimbal ring. What you want to do is have a helper hold the steering wheel tight, and you want to go and try to move the sterndrive side to side. You are allowed a little bit of slop, about 1 inch off of center. But any more than that is to much and indicates a worn out gimbal ring. Click this link below to pull up a video, this one here would have to much slop.
If that looks good. And If you brought the boat to me (and the gimbal ring looked good to me) I would go ahead and replace the ujoints. They can feed "tight" by hand and appear to be good, but trying to wiggle something by hand and putting 140 horsepower at a few thousand rpms through it are 2 different things. So if the gimbal ends up looking good and the steering is tight, replace the ujoints.
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