I sail a J34c with an original (23-yr-old) Volvo----starts easily and runs well----except that occasionally when I first shift to forward and increase rpm to move there is a loud vibration noise, sometimes with a knocking sound, that increases as i increase rpm-----corrects when I return to neutral, increase rpm to 9-10000 and return to forward. The prop is a Martec 2-bladed folding. The cutlass bearing was replaced this winter and the boat has been in the water for only a week, so not a barnacle problem. At idle the engine is at 7500-8000 rpm and is rough---smoothes out at 8500 rpm. Any ideas about what's going on?-----Thanks