Yes those are your engine mounts in the picture. They more than likely bolt through a steel
plate mount, through a rubber vibration mount into a recessed threaded female bolt that is glassed into the hull
. The loud knocking is definitely more likely your injection pump or even more prevalent on older Yanmars is carbon build up and misspray from the diesel
injectors. This would make your piston fire incorrectly and create that loud knocking and vibration. I would have a Yanmar mechanic
come and check it out. Another way to do a basic troubleshoot is to fire up the engine and if you have white or black smoke coming out and/or a sheen of diesel on the water
surface next to your wet exhaust
, you probably have an injector problem. The good news is the cost to fix is relatively inexpensive, I think the last time I checked on our Yanmar it was $62.00 to replace the ejector needle valves or $50 per injector to have rebuilt. Good luck and keep us posted on the problem and solution.