I would think that it is just simply a resonance factor.
Ever heard one of those old Cummins 160's?
Let them idle & they are as quiet & smooth as a church mouse. Increase the
revs very slowly & you'll find that at 1400, 1600, 1800 & 2000 revs they
just about shake out of the chasis. (truck)
Most diesels do it to a certain extent. Next time it does it, try adjusting
your governer up or down a few revs & see how it goes.
My guess is that under that certain load, it hits the "rattle" speed.
Maybe if you can tweak it a weeny bit, it might not come down to that rattle
speed under load?
BruceM


"Wayne.B" wrote in message
...
On Sun, 5 Sep 2004 09:49:03 +0200, "Steve Lusardi"
wrote:
My guess is that it is not electrical. I think your flex plate drive is
either loose or worn out. The alternator has bearings only on the
outboard
end. The engine rear main bearing acts as the other armature bearing.
This
mechanical connection is usually done through a flex plate which is
bolted
to the engine flywheel and a splined hub drives the armature or rotor. To
check this, the alternator must be removed from the engine.

===============================================

Thanks, your assessment of not being electrical jibes with my
observations. What would explain the intermittent nature of the
vibration given a more or less constant electrical load? Is the flex
plate a coupling with a rubber hub?