Catalina 350 International Association

Ours is a 2003 model with ~850 engine hours. After a 100 engine hour trip to ME and back in August I am hearing a rattling or grinding noise when in gear at around 65% of full throttle that gets louder as I bring it up to 80%. No noise when run at high RPMs in neutral, and no vibration when in gear or neutral. No problems with the transmission. It sounds like it's coming from the muffler/silencer but I see no posts about that. From what I read it doesn't seem to be the damper plate, but I am not sure. Any guesses what might be causing the loud and troublesome noise?

Ya, dampner plate issues usually show up at lower RPM in gear.

I would start by retorquing the engine mount to stringer bolts and engine mount to engine bolts. Actually, I see this as part of annual maintenance.

Also, check tension on the belt and bolts that secure the alternator.

After that, can you have someone else drive and you remove the engine cover and see if you can further locate the source.

On my 2004 C-350 I have had a lot of vibration as I increased throttle; really vibrated the boat. For a long time I dealt with this by easing off slightly after accelerating which calmed things down.
Many repeated checks; shaft alignment, engine mounts, prop, bushings, rudder- on and on, finding nothing amiss. Just recently noticed that when I was in a somewhat tight turn, either to stbd or port- no sound, no vibration. I have a 3 blade fixed prop. The rudder has no play. So what seems to be happening is the prop wash over a dead-ahead wheel is what is shaking things. Is this something others owners have noticed ?

I get a rattle/tapping sound coming largely through the port side aft hull, which comes from the muffler, the one way gate at the entry point (I think) causes it. I have been planing on changing out the lift muffler for years but haven't got around to it.

My experience with vibration has been the huge difficulty of getting perfect alignment and a perfectly balanced prop, I sort of suspect that maybe the vibration is from your prop/shaft combination being out of balance, but when you are turning sideways forces on it press it to one side or other of the bearings/cutlass bearing and the vibration diminishes.

Clicking noises like a badly set tappet on a petrol engine could well be the lift muffler, so have a close listen to that.