Here is a good way to give the group a good chance at helping me with this .. Watch this video from this weekend. The very end is a shot of me coming toward the cam then I let off the gas and the sound is loud ..
http://www.isphub.net/wayne/day-in-boat.wmv
It just started when I took prop off and tried a new one, then put old 25p back on for these pics
thanks

I cant say for sure, but it sounds like the rst of the i/o's on my lake running prop exhaust. I hear them whine like that at speed. Yours was heard when the exhaust got quiet.

Maybe a bad gimble bearing?

Yea if it had not come on so fast I would not have thought anything of it. But when you can hear it over the thru-Hull exhaust it must be load. May be unrelated but the speedo quit working at same time.

I am no drive expert, I am sure some more knowledgeable guys will chime in but it sounds like a gear noise caused by the back loading of the gear sets. I would say not a Gimbel but that is a cheap easy try. Bevel gears (like in the top and bottom of a drive) are not always quiet and that is not always a bad thing. Every boat I have wake boarded behind with an alpha made some whine under load. How many hrs on the drive and what drive is it? If the backlash and preload is not setup right bevel gears will make noises to tell you they are unhappy but it sounds like they are OK in there normal condition. No drive noise while underpower? I now hard to tell with thru hulls.

It is a bravo 1 with 120 hrs. No work ever done to it. Oh yea starts at 2500 and goes to about 3000 rpm then get quiet after that. does not change much with throttle position.
Thanks for chiming in !

What does the oil and magnetic drain plugs look like? A bit of "dust" is normal, but sure no chunks.

Looks clean . Thank though gives me a reason to add fresh oil