Little knocking sound when giving a little throttle

[jjuriga]

Started up the boat yesterday outside on the trailer... to get ready for today and noticed a little knocking sound while giving some throttle. It does not sound like it's coming from the engine... oil pressure good, checked the oil level myself. All good. (200/v8 merc)

I did notice that if I engage the throttle and the propeller spins a bit, knocking stops immediately.

Any ideas? I have had the boat out and running several times since finishing the rehab project and have not heard this sound. Thanks in advance!!