Opti noise (clatter?) question

[Mr. Demeanor]

Every now and then when I am plowing water I hear this kind of clattering noise. Almost sounds like a bad knock. Its is only when the engine is kind of lugging and I am plowing water. It sounds bad but I have been through the entire engine. The compressor was recently replaced and I thought it may have been that when it was going but its still there. Pulley/belt tensioner have been checked and are good.
If I change the trim a little it goes away which leads me away from it being knock. Only seems to do it when trimmed fully in. A big load in the stern seems to bring it on but its inconsistent. I think it might be water slapping the lower cowling or something like that. Sounds strange but it does sound like it could be something like that. My boat has a strange vented notch setup that could be causing some weird turbulence. Maybe the exhaust is getting submerged causing a strange sound???? It sounds bad though when it happens. I probably need to get someone to drive and hang over the back and try to narrow the source down.

[Mr. Demeanor]

Just one more thing, I have never heard a 2 stroke knock. Is that very audible becasue this is pretty loud.

[Capt.Insane-o]

What RPM?

[E-tec1]

check the reeds????? ive seen many of them lose a reed or a few, it sounded like a knock, just a suggestion

[Mr. Demeanor]

I tried all day today to reproduce the noise. Couldnt do it. I actually had a heavier load on board and purposely loaded the motor and nothing...

[XstreamVking]

Silly question for ya... check to see if the props on tight? Have had loose props make noises when going slow and slightly venting some air.... Under load goes away! Jus a thought...

[Mr. Demeanor]

Ive put quite a few hours on since the original post and it hasnt made that noise since. Not the prop either.

[sosmerc]

I have noticed that when trimmed fully "in" alot of outboards will develop alot more noise. This is because the swivel bracket is solid against the transom bracket. If you trim "up" just a touch...the noise level goes away. Some heavy stainless props at low idle speeds in gear will develop a noise we call "clutch chatter"....it is even more evident when trimmed in fully. The "chatter" can be quite pronounced and sounds like something is about to come apart

[rock]

I run an xs motor on my Allison. It does sound different than the older ones. We fish a couple times a year and when I do it is usually with a Ford mechanic friend of mine. The first thing he said after changing to the xs was "what is that knocking sound?" I think it is just a different sound than what we're used to. From my experience with 2 stroke motors, if it's knocking it won't be for long. This xs is 6 years old and still running strong.
Rock

[stokernick]

sounds more like my wife's sewing machine to me,but I love it!