rattle in mercruiser 140??

I just bought another project boat that has a 1985 140 merc 4cyl. this engine was rebuilt 2 years ago and the boat has only 10-15hrs on the new engine. it has what I will call a Rattle in it. all I can tell is it is not from the topend! pulling the plug wires does nothing to change the noise and it is more noticable on idle then at higher RPM's. when you shut the engine off it changes tone a bit and gives one "more of a clangy" noise just before the engine stops?
the engine does have a slight miss and some hesitation giving it throttle but over all seems to run well once warm.
I know this is not recomended but I had a bottle of STP oil treatment (like Honey) and I added it to see if the noise would change, typically it will change or quit if it is lifters or bearings, this also made no change at all in the noise, (I will change the oil, don't worry)
so I am wondering what other sources of rattle are their in the 140 mercruiser? i would not really call it a knock and by using a screwdriver in your ear I can't pinpoint it at all except you hear nothing from the valve cover. and it sounds more from the rear of the engine.

this engine was totally rebuilt by a local shop with new pistons, rings bearings etc...

Re: rattle in mercruiser 140??

after spending the last hour searching past posts, I have found a few possibilities.
1st would be piston slap? seems like this can be common in newly rebuilt 140's. and my noise seems to get lost in other engine noise at higher RPM,s but is their a way to confirm this?

2, loose flywheel since it was rebuilt this could be possible

3, bad gimble bearing.

4, rod bearing, but am I right is concluding this is less likely since it is worse on idle, and is less noticable at higher RPM's.

i hope to dig into this tonight or fri Am so any suggestions would be helpfull

Re: rattle in mercruiser 140??

well just played with the boat again, let it run for 20+ minutes looking for the source of the rattle with a stethescope. thought it may have been the #4 lifter but then I removed the plug wire and this time it deffinatly changed the noise to almost gone! before the engine was cold and it made almost no difference. but now with a warm engine it does? so would this still be wrist pin? or still possibly piston slap?
you can hear the rattle thru the removable cover like it is a bad lifter, but no noise thu the lower block and no noise in the topend