I have a 4.3 Merc and a Bravo three drive how can I tell when the rear mounds are worn. I have this knocking noise that comes on now and then. I have replace the gimbal ring and steering shaft I have also check my aligment and Its ok. Dont know what going on.

If the noise you are describing is only in neutral, it is normal. What you are hearing is a bit of backlash in the upper drive gears when they are unloaded.

Kurt

When you have the drive off check the gimbal bearing by turning it by hand if it has any rough spots and doesnt spin smoothly it should be replaced. also when the u-joints are going bad they knock, usually when the drive is turned fully to either side it sounds worse.
good luck