reverse knocking

[phester]

that happened to a Mercury I had, the forward and reverse gears were fine but on the back of each gear are the clutch dogs, and in forward, just a bit over idle speed it sounded and felt like I was driving the boat over a bunch of rocks....not a good feeling. Step20, I hope it's the case, but , if it is, you already need a lower unit [ the reverse gear, that is]