15 HP 2002 Merc 4 stroke - will the powerhead from a 2008 interchange?

Greetings all, newbie here.
I've a 15 hp 2002 Merc 4 stroke used as a kicker on my sailboat, when it first starts it produces a noticeable knocking noise and then ceases I'm assuming when it gets oil. It runs fine even on runs of 15 miles. I've been around engines for the past 40 year and my instinct is a worn rod - sounds like just one, not both.
I can purchase a 2008 power-head that looks to be identical. Does anyone here know for sure if it will interchange?

Anyone hear of rods going bad on these motors?

Thanks in advance
Jan

These have a mechanical fuel pump that makes noise when it has no fuel. Most likely that is what you are hearing, try disconnecting the fuel connector while it is runnig and wait, when you hear the mechanical noise tell me if thats the same as your knock.

Thanks so much for the info, I'll check that out as soon as I can. Currently the boat is on hard but with a little work I can fasten a container on the bottom of the engine and start it. I sure hope that's what it is. Is there a cure for that other than an electric fuel pump?

I just thought of something, I always run the engine out of fuel after a day out sailing and have never noticed the knock then, but I was not really listening for it. I will check what you suggest - and hope you are correct regarding the fuel pump. If it is a worn rod have there been many of these reported?