Slight knock, 2003 DF 140

[Pfd161]

Have put about 20 hours on this boat and it runs great , total hours around 180.
Last time out could hear a slight knock, at trolling speed. Seems to be from the top of the motor under the cowling like flywheel?

Yesterday it did it for a couple hours then went away. Also, doesn't do it at all when I throttle up a little . Thoughts ?

Thanks .