I recently replaced my Mercrusier Alpha1 Stern Drive with a genuine new Mercruiser unit.
It seems to run fine but there is a kind of harmonic humming noise at cruise speed, 3000-3500rpm.
I broke the unit in as per mechanics guidelines and have replaced the break-in oil. I have put about 40 hours on it since replacement. Gimble Bearing was checked and is in good shape. I am using a Merc aluminum 16x16 prop which has some cavitation burn marks on it but is otherwise in good shape. I did not have this noise with my original leg. Any ideas what may be causing this and if I should be concerned?