had a few boats that did this over the years and as zabooda calls out, usually due to having the rudder turned while fully loaded up and heavy acceleration.
if it's happening every time, might check the blade for a ding, rudder or shaft play. had a blade that hit a rock and had a slight vibration to it until fixed that was similar to cavitation and would vary with rpm becoming more and less noticeable as I moved through the rpm range.