what kind of exhaust are you running

Quick and quiet corsa

Could it be the internal flappers inside of the exhaust making the noise ?

I had corsa on mine, the internal flappers made a lot of noise at idle. It was a very loud ticking noise. It sounded like the valve train was coming apart. You can put your hand on the pipe at idle and feel the flappers are moving with the same pulse as the noises.

you certainly can hear the exhaust ticking but you can feel the noise when you put your hand on the outdrive top cap we have eliminated the possibility of it being in the outdrive by trying a known good outdrive and same noise I have used mechanic stethoscope to listen to exhaust bell housing cant hear it when I am in the engine compartment only at rear of boat on the hose at 100 rpms or higher and when I turn the wheel in either direction it gets worse
I know sounds like u joints but changed mine first brand new mercruiser parts no change then when we put the other outdrive on and same noise we confirmed it was not u joints
Thanks for all the comments guys this thing is driving me and my dealer crazy it is going back in this week we are going to check the rear mounts as a member has sugested

Does the noise go away when you put the drive in gear? If so you are hearing the shift fork cam bouncing up and down as the bump rings spin. That space will move up and down as the gears turn, causing the shift fork to bounce up and down. You can feel it and hear it. First time I heard it I was very concerned. Next time, not so much.. then I ignored it. Never had a problem with it. If you can find nothing else amiss, this is probably what you are hearing and feeling.
Hope that helps..
Dick

thats very interesting sound does change when put in gear but still there would this sound still be there even when we changed drives to a known quiet drive? I have tried playing with the shiftier while it was running not quite putting it in gear and the sound still remains constant

Try to run it with the drive off, if you have a water hose connection plumbed up. I don't think the noise your hearing is the flappers in the exhaust but one way to eliminate that noise is to take two broom handles and hold the flappers still while the engine is running. Keep us posted with what you find.

sounds like a plan thanks to everyone on the board for your help

Did you compare it to another boat connected to a water hose to see if they sound the same.