Freshly rebuilt engine developing knock

[Weathersbytaylor]

After finding a wiped cam lobe, I went in and rebuilt the entire engine (new pistons, bearings, etc). Engine has roughly 10 hours on it total now including about 4 hours blasting around the lake. Completed the break in oil change and noticed that the engine was developing a low end knock. Didn't hear it before the full day at the lake. It appears a few minutes after starting the engine and is noticeable until about 2500 rpm when it's drowned out by engine noise. It's not as loud as other engines with spun rods that I have heard and is not noticeable when the engine compartment door is shut. Pulled #4 spark plug and the noise goes away. I'm thinking I may have caught whatever the issue is fairly early. My question is how likely is it that a rod has spun a bearing? any other things I should check while the pan is off? Gonna pull the pan tonight to find out. Oil filter didn't have any observable metal in it. What's the easiest way to differentiate piston slap vs rod knock? Thanks for any input guys

1996 mercruiser 3.0lx.

 

[09TNRT]

Piston slap is more a "light" sound of that makes sense. More like a clatter. Rod bearing sounds more uh " meaty" lol. Anyway you could post a decent video to hear the noise?

 

[Rick Stephens]

Time to pull it down and check it out before you have a bigger mess.

 

[alldodge]

If its a knock, you need to start again, if its a tick you can try to figure it out

 

[dusted_ya]

If you hit a rubber mallet on a piece of wood that's sort of how a rod knock sounds. Pull the suspected valve cover and check your top end first for maybe a collapsed lifter or something. If you suspect something in the lower end you'd better pull the motor. I've had flywheel bolts come loose and make noise like a rod knock though.

 

[09TNRT]

Dusted_ya is right on the flywheels bolts. My buddies Audi did it and where the flywheel had been just a couple thousandths loose is messed the crank surface up good. I also just pulled the trans in my neighbors jeep and out 2 months ago for the same problem. I would probably just pull the motor back out. You've done it once so it should be much easier this time around. That way you're not leaving anything to chance. Maybe a bolt just didn't get torqued right, sure wouldn't be the first lol.