Took my boat out for the third time Saturday. First two times it ran great. Saturday, I had been going for a few minutes when I started to feel a pretty serious vibration. Did not hit anything. I stopped for a few minutes and looked around best I could (too cold to get in). I started back up and it ran fine. Ran several miles down the lake and back at 30-40 mph with no problems. As I was pulling back into the cove where the ramp was, it started vibrating slightly again. Not as bad as before. Looked closely at the prop and shaft when I got it on the trailer and did not see anything unusual. Any thoughts?