Went for a ride in a friends boat with twin 575hp 540ci mercs with 270 hours or so. Port side motor started clattering around 3200 rpm. Lots of oil pressure, sounded like valve train clatter of some sort. Limped back to shore with port side motor clattering at a gradually lower rpm then 3200 by the time we got to the dock. What are some opinions or suspicions on this issue?

pop the valve covers and take a look for something loose or valve stuck down.and take pics for us.