Is this noise normal?

[scarmack]

I have a 1998 Bayliner 205 powered by a Mercruiser 305 with the Alpha One sterndrive. I just purchased this boat and the powerpack seems solid yet while underway at cruising speed (say 3000 rpms) the outdrive seems to change pitches at times and not necessarily with any particular trim locations. I had an OMC for years that hummed a constant pitch and am just familiarizing myself with the personality of the mercruisers. Dont get me wrong, its not a drastic change in pitch but it is noticable. It is intermitent and I am faintly concerned. Is this common with mercruiser outdrives or is this an indication that something is out of line.
I appreciate anyones comments with experience here.

Thanks,
Scott