The last time I heard noise like that (3 cyl two stroke outboard mercury) it turned out to be piston slap. At idle was quite off idle at about 1500 on up accelerating could here it no problom. Also could here it decelerating until back at idle again. I figured it was #1 due to scoffed piston (oil injection failed) even with new piston & rings at TDC I could rock the piston with an all or screwdriver back & forth alot! & if I did it fast enough could almost duplicate the noise. #2 & 3 were notably tighter in there bores. Id make sense because it would make a "rattle" every time that offending cylinder would fire making p.s.I. spike rocking itIin its bore. Is it any worse cold or hot?
