Went and looked at a 2000 Donzi 38ZX this past weekend with about 378 hours on motors and drives. Motors haven't been touched except for valve springs replaced at 250hrs. When The owner started the motors up in the driveway on the hose there was a knocking sound that he said was coming from where the drive meets the motors and said that it was a normal sound while there was no load on the drives. It wasn't consistent, more random. Both motors/drives did it while idling and drives were in the down position. He said when there in gear the noise goes away, in fact I believe the noise went away when he rev'd them too but who know as the motors were louder and you couldn't hear much more.

To describe the sound it was like a knocking sound that I thought was coming from the motors but like I said it wasn't consistent, he pointed it out before I said anything about it. I think he describe the noise as where the motor meets the drive it kind of free floats and that when you get that noise, I don't know. Does anyone else have the same issue.

Coupler?

Universal Joint?

Probably just the backlash in the upper gear set. More pronounced in an xr with straight cut gears.

Unless you have extension boxes, where the motor meets the drive is in a splined connection at the coupler. There is no, or should I say, there should be no slop in that connection to cause a knocking sound .

Internal flappers make a knocking sound at idle

Didn't think of that Jason.

How were the gimbal rings? Did they have slop or have they been recently replaced? If they had been replaced, the previous sloppy action could have ate up the engine coupler.

Not sure on the gimbals. Aren't the flapper rubber coated? So this sound isn't normal? The guy seems like he takes care of everything the boat is flawless.

were the drives down ,gimble barring