Vibration ?? Rumble while in forward gear, low speed

[FishFinder]

During idle speed (just in forward gear) I sometimes get a vibration (rumble) coming from the engine.. It’s really kind of hard to describe, almost sounds like its coming from the prop?? But if I bump the throttle up a tad bit the rumble goes away.. Any idea?

2005 F225 15hours on it..

[boats.net]

The large diameter stainless steel props will sometimes cause a tapping noise that is caused by the drive shaft moving up and down at idle, you don’t hear it with a aluminum prop, as soon as you speed up centrifugal force will hold drive shaft up and the noise will go away. It won’t hurt anything but is annoying. Remove the prop and check that all thrust washers are in place and prop is tight. Have the idle speed checked it should idle at 650rpm in gear, if too low then adjust idle speed.