Thumping from a 200 hp 4 stroke

[aquadj]

I just bought a boat with a 2004 Yamaha 200hp 4stroke motor with 20 hours. Everything checked out when the boat was surveyed and motor inspected. They even did a computer scan to confirmed hours, and normal compression test, etc.

During the first time using the boat, I heard a thumping sound while I was docking. It was a sound similar to what my 2 stroke motor made when it "rattled" at idle, but this 4 stroke has no rattle (nice and smooth). It doesn't sound like a metal to metal grinding, more of a thump at a rate of 200 times per minute. It doesn't always happen and there isn't anything specific that I can do to cause it.

Since I'm new to 4 strokes, I'm wondering if this noise is typical with 200 hp 4 strokes or should I be looking further? I know the motor was winterized prior to a northeast winter storage, but I have very little additional service info. I'm also assuming the fuel (ethanol free) has been in the boat for 2 seasons.

[boats.net]

It could be a prop rattle(nothing to worry about), if it goes away when speed up or in neutral then it is probably prop rattle.

[smalldog]

If the water pump was changed and they did not lift up on the drive shaft when they put on the anti knock stuff on top of the impeller last time it was changed... it will knock at an idle.
The cut of the gear lifts the driveshaft rather than pull it down and because of this...you need the anti knock stuff and it has to have the slop taken out when installed or it bounces up and down with no load.