I have a 5.7 350 Magnum Engine on an Alpha GenII Mercruiser outdrive. There is a knocking in the outdrive when going over 20 mph. There is no noise at all when going under 20 mph. Knocking is not in the engine. I checked the u joints and they were tight. There is no metal shavings in lower unit oil. Any ideas what could be the problem?

Thanks!

Something is loose and the torque at speed is causing the knocking...Are you sure its the outdrive? Could it be your flywheel? Bellhousing?