Transmission whine

[Sepplino]

Bought a 1994 Ski Nautique recently. The boat was sitting in a garage for 10 years and has only 300 hours. Unfortunately the transmission whines heavely.
I also have another 1994 Ski Nautique (both are exactaely the same configuration) with almost 1500 hours on it and the transmission does not whine at all. Already checked the alignment of motor and shaft, but that is fine.
Any ideas ?
Thanks for your help, guys !

[SilentSeven]

Same prop? There are some boating threads that discuss prop singing as a phenomena. Also, trust you've flushed the fluid? Outside of the noise...does it work ok?

[Sepplino]

Yes, same prop (ACME 422). Outside of the noise it works perfect. Yes, have changed the Oil (DexIII)

[functionoverfashion]

Can you post a picture of both transmissions? I'm curious about something, if it's not too much of a pain.

On my '89 there was a superfluous little hose that came up off the transmission. If you removed it, there was a terrible whine (I tried once, and had to fish mine out of a dumpster). I don't know if your boat would still have that, though.