Went out in a buddies new boat yesterday, would rather not say what make, it has a pair of 525's with the ITS set up. Nice set up and runs fine but there seems to be a noise coming from the engine compartment, sounds like water slapping against the hull or a broken u joint. Is a unconventional noise normal with this setup.

Did you open up the hatches and check anything? Do noises stop when idling?

yes I investigated the best I could the noise is there in gear and out of gear at idle

could be the internal flapper in the thru hull exhaust. If it has thru hull exhaust and it doesn't have the rubber flappers on the outside they are prob. internal. They will make all kinds of noise on some brands. If your boat has mufflers or baffles in the pipes it's prob. something else.

it's the internal flappers, they make a freakin racket

I gave the flappers consideration and then forgot all about them . I will check them the next time we go out. THANKS for the input much appreciated