Knocking noise from engine when starting
 

I started my boat the other day and noticed a knocking sound coming from the engine. When I lifted the engine cover I noticed that the knocking seemed to be coming from the front of the engine, but it did not seem to be related to the rotation. By that I mean that the knock occurred about 1 every 1/2 second. It's possible that it's related to one revolution of the serpentine belt. Definitely not once per crankshaft revolution. As I tried to localize it more it faded away and I couldn't reproduce it. Boat ran great the rest of the day. Water temp, oil pressure, battery all where they are supposed to be. Oil is at full line on dip stick.

i started it again last night at home and it again did the knocking, but faded quickly. I pulled the serpentine belt off and ran it for 5 seconds it but there was no noise. I spun the alternator, water pump, impeller pump and 2 idler pulleys but I didn't hear the noise. The one idler pulley seems to have a bit of play in it, but nothing that would strike me as being bad.

It's a 2002 Mercury 350 MAG MPI engine.

Anyone have any ideas on what it could be?

Much thanks.

Trash