[C320-list] Grinding noise while motor sailing?

[JZ]

Hello all, Yesterday i was motor sailing with just my headsail up. I was doing about 7.5knots, engine at my usual cruise rpm of about 2800, all of a sudden there was a grinding/rattling type noise and a slight vibration. I immediately slowed the engine to idle and put it in neutral, the noise stopped as i slowed the engine. I then put it back in forward, no problem, gradually increased rpms to 2800 again, and no problem.  I am baffled as to what it might have been? It was fairly rough out, 4-5ft sea’s, we were battling a head current trying to get to our intended destination, hence the motor sailing. We were heeled over approx 15deg. Once we were anchored, i went below and checked the engine/trans area, everything was tight and secure. I checked all the fluids, normal. Trans oil was bang on and nice and clean.

Anyone ever run into anything like this?

It is the Yanmar 3gm30f with the Kanzaki trans, approx 2500 hours.

Thanks!

Jeff Zinselmeyer
s/v Escape #934