ok, this may sound a bit far fetched but it happened to me. Check to make sure the throttle cable isn't touching the alt belt. We were cruising across the lake when I felt a little vibration on my foot and was hearing a weird ass noise. The throttle cable was just barely touching the belt making the noise and vibrating through the cable.
In any case, hopefully it's something simple!
checked the cable and no such luck, but the idea deffinately made sense...oh well, at least its the end of the season