Grinding noise in reverse

[bostongrun]

I took my 2006 Glastron GT185, 4.3 Penta Volvo out yesterday for the first time this year. About an hour into using it, I starting to hear a kind of grinding type noise from the lower unit, especialliy in reverse at higher RPM. No noise at low rpms.

Not much use on it, probably less than 100 hours; any ideas. I am resonably good wrencher, I have reoved lower units to do water pumps, installed starters.