This may or may not be the problem but I just went through a rattling with a
customer's Yamaha. The power head was just replaced 17 hours ago. The noise
was more evident from the bottom of the power head down to the gearcase more
than in the upper engine area. I ran the engine with the gearcase removed
for a half minute but the noise remained. The customer reported that
performance didn't seem to be affected.

The problem was that the torsion dampener attached to the bottom of the
crankshaft came loose. I checked with Yamaha tech services about this and
they reported hearing of the problem several times. The fix was easy enough,
about 5 hours work and there was no damage from this happening.


Tom" wrote in message
...
I have a new Yamaha 200 (ox66) outboard with just 22 hours on it. The
other
day I took it out and noticed a slight metallic rattling noise while it
was
idling, seemed to go away after a few minutes, then while running about
3600rpm's I heard a loud thud coming from the engine, no change in
performance just a loud thud... I stopped and checked everything over and
then continued, every so often we would hear this same noise..can't find
anything obviously wrong..just looking for some ideas before I take it
back
to the dealer. Thanks... Tom