Try revving it higher in neutral. Well into the range it makes noise. 1300 rpm isn't much. Don't go crazy, you don't need 5000 rpm. If no noise, then that probably isolates the problem as something aft of the motor
, coupling, stuffing box, shaft log, etc. Though not necessarily. And some or all of those things may not make noise until and unless under load when the prop is pushing it all forward and maybe to the side if out of alignment.
It's talking to you now. You better figure out what language it's speaking before it tears up completely, possibly taking a bunch of other expensive stuff with it.
And are you sure it's not the belt???
Eliminate that first, one way or another. Have your wife run the boat until it makes the noise. Spray WD40 on the underside of the belt. Between the belt and pulley. If the noise changes significantly you have found the problem. The wd40 will wear off quickly and it won't hurt anything.
A belt may not make dust until it is about to fail. They make dust when over stressed. As with a high output alternator under heavy load with an inadequate belt/pulley combo. High output alternators usually run a double belt system for that reason.
Belts can also become glazed if they are just slipping from being under tightened. If the underside of the belt has a shiny smooth appearance, it won't grip the pulley. And once glazed, no amount of tightening will make it work.
Why not just change it out now? And don't just get the same belt that's on it. It may not be correct. Study up on how to properly tension it. It may need to be tighter than you think. But too tight and you can burn up bushings in pumps, alternators and such.
Water output is no indication of belt tightness. Nor is charge rate.
An alternator belt might screech on start up if the battery
is discharged from cranking and the motor
is revved until it enters its high charge RPM range. But your regulator
may be set not to produce high charge rates at low RPM so as not to stall the motor. It could be the belt is screeching when the alternator reaches full output RPM. Which is probably around that 1500 range.
Or.... you can just wait and see what breaks!
This might be a good time to assess your mechanical abilities. Sometimes, it is cheaper to call in a pro early.
You're in Ontario
? Are you hauling out for the winter? You may want to at least diagnose it before you haul. Mechanics can get pretty busy in the spring.