Dude, I'm guiding your testing based on my very similar experience to your shutdown description including the squeal (see links to pertinent posts below). This may turn out to be something different but if so, the testing will eliminate my hypothesis which is good information too. And by the way, this thread has reminded me it's time to replace my packing flax.
That said, I don't mean to discount your observation that the squeal came from the front of the engine but frankly when the engine is running it's hard to pinpoint the source of noises. Also, your successful test with the driveline disconnected points me to the driveline and away from anything going on at the front of the engine. Let's get the second test done (driveline reconnected, stuffing box adjusted loose) and see what it tells us before we start chasing other zephyrs.
Looking forward to your next report.
https://www.moyermarineforum.com/for...5&postcount=14
https://www.moyermarineforum.com/for...0&postcount=19
https://www.moyermarineforum.com/for...5&postcount=11