The GM series Yanmar has an operating RPM of 2900. Operating at 3400 RPM for 3 hrs. is a little much.
However, I have had a similiar issue with the warning horn, after I got pooped repeatedly, and submersed it. It kept chirping at very low volume, and intermittently for a few days, until it dried out. If everything ( temp/ press.) seems ok, then get a new beeper.