After you ran it at higher rpm on the water for a while, did it still make the noise later in the day when you idle down, or did it go away.

A slight tight spot in the guide which only makes a little screach/chirp at idle when the valve is opening and closing slowly. What do you have bronze guides and inconels with PC seals?

you hear it under hatch or outside the exhaust behind boat?

My chirping noise turned out to be the same as Turbo's. You didn't say what engine that you had but mine was a 1997 500HP. One of the roller lifters failed due to the roller 'skidding' on the cam. The roller and pin eventually wore down until the 'ears' of the lifter were scraping on the cam (hence the chirping noise). I had to replace all of the lifters and cam as it scarfed up the lobe that it was riding on. I had heard that this was a common problem with this vintage of HP's especially if you use synthetic oil as it is 'too slippery' for the surface finish on the cam lobes to make the rollers spin 100% of the time. When I checked the other engine, one of the lifters was beginning to do the same thing but I caught it before it damaged that cam. Needless to say, I no longer run synthetic oil in these engines....

Wouldnt that be a pretty significant reduction in power. Did you notice the power loss?

I get this on my motor when engine speed drops below idle a bit when the IAC motor doesn't respond fast enough. I think it is just the tips and flappers as well. Goes away as soon as revs return.

your lifter did not break because the oil was to slick,it failed because the valve springs were weak and the lifter was not being kept in contact with the cam lobe.lots of people using synthetic oil with roller lifters without any problems.do you really think it is posible to have oil that is to slick?

My thoughts also, I love tech posts lol

Yes, it consistently makes the noise at idle. Pretty much goes away at anything above 1000 RPM's

Is the noise heard under hatch or out the back of the boat?