Engine/Transmission rattle

[snewman]

I noticed a rattle at low rpms that seems to be coming from the flex plate between the engine and transmission. At higher RPMs the rattle dissapears.
"Quo Vadis" is a 1987 27' Albin family cruiser, Hurst Transmission, 78HP Nissan Engine.
I have not owned Quo Vadis long enough to know whether this is normal or signs of trouble coming. Any thoughts?
Thanks,
Scott

I posted this a couple of weeks ago and no responses so I thot I'd try again to see if anyone might know what the rattle might be.
Scott

[jrw]

I had a rattle that sounded like transmission or flywheel. I was told it was the springs on the flex plate that rattled when going at idle speed. I also had a slight vibration in the shaft or wheel. Upon changing the prop, the rattling disappeared. You might check the shaft, shaft alignment and prop to make sure they are all proper. Hope this helps.

[jleonard]

A rattle at idle rpm is common esp if the idle is in the low end of the range.
The springs in the damper plate don't stay loaded and that loading/unloading is the rattle.
Try kicking the idle up by 25 or 50 rpm and that should make it quiet.

[jointventure]

I had no rattle but discovered hard pieces of rubber (similar to the raw water impeller) in the bilge. Determined it was the damper assembly between the engine and tranny. I have a Cummins 450hp with a ZF280 tranny-the damper consists of two pieces,(an aluminum plate which mounts to the flywheel and a heavy,robust steel plate splined to go over the tranny shaft) which are vulcanized together to form one assembly. The inner part of the assembly has "soft" rubber (the pieces in the bilge) which dampen the vibrations at idle speed, the outer part has harder rubber for normal operation. The failure of the soft rubber will cause a "clunk" when shifting and possible vibration at idle speed,however, it will not affect normal operation.
The repair consists of disconnecting the shaft coupling and sliding the shaft back,blocking the back of the engine up-the rear motor mounts are on the tranny-,disconnecting the tranny from the bell housing,move the tranny back and with just enough hand room remove the failed assembly. Without enough room the engine would have to have been moved forward or pulled.I was lucky!!
Total cost $2100.