That squealing noise is a bearing on the bottom of the powerhead.A friend of mine has a 1997 90HP that does that.Only after extended full-throttle running in warmer weather.Could be a design flaw that does not let enough oil to that bearing at full load.That is what one Merc tech told him.
If you pulled the powerhead,it is the bearing assembly that the driveshaft goes into.Have not heard of any that grenaded,so he says”party ’till it pukes”.I heard it twice,and it can be hard to pinpoint.His has been doing that for awhile now.
It is a roller bearing,so it could sieze if it got too hot.Not sure what extra oil will do to help,but couldn’t hurt.