http://img856.imageshack.us/img856/3...gjqsznnboc.mp4

That link should take you to a quick video.. Both motors sound identical when they are in neutral, there's no rattle. And above 900rpm theres no rattle. But at idle it sounds like a dang ducati!

I've never had these before.. They're on a 8.2MPI in a Formula 41PC..

Is it the clutch plates rattling? Are they supposed to??

another quick video..

http://img802.imageshack.us/img802/6...smsuvlagbc.mp4

My Twin Discs do the same thing. I purchased new PSI brand damper plates and it improved a lot, but it is still irritating as hell. I raised the idle to 850 on my 502efi's using the mefiburn. The noise was all but gone. Unfortunately the engines were hard starting when warm, so I had put the idle back to 650 until I have more time to fiddle with the starting issue.

Is it coming from the tailstock of the transmission or from the bellhousing area?

That doesn't sound good at all.

That is pretty normal for those. They have a big reduction gear, you are hearing the backlash. It is possible the drive plate on the back of the engine is the wrong one or has worn out. A rough idle will contribute to this also.

sounds like the drive plate springs are weak..won't really hurt anything...a lot of inboard boats do it for years...if it goes away by increasing the rpms slightly thats usually what it is...nothing in trans can do that..

It can also be the splines in the slip joint of the driveshaft and the lash in the drive.

If I bump the RPM up slightly, it goes away.. The tach's are showing 600rpm at idle and seem to be idling pretty smoothly.. Doesn't do it in either gear, just at idle..

Thanks for the help, hope its not as serious as it SOUNDS! Both trannies sound identical in neutral..

These are 2.455:1 also

Sounds like a dampner. Just had to do that and a rebuild on the VD in a ski boat.

It is backlash. If you have a low idle (which it sounds like you do), a slight misfire or sketchy plug wires, you are going to hear backlash in your tranny. The only real fix is to give your self a tune-up and make sure everything is 100%, or bump up your idle to 750.

On my last Sea Ray where I had dual Velvet Drives, I ran through this entire scenerio. I mistakenly had the trannys rebuilt and new flexplates installed first. Still had the same noise in both. Gave it a tune-up and the noises went away for the most part. My idle was at 500 at that time. I bumped it up the recommended 700-750 and the noise went away completely. Treated me well for another 800 hours of use until I got rid of the boat.

Although you may be putting a fractional amount of wear on your gears, you are not doing any real damage. It really is just more annoying then anything else.