When under stress, the goodies between the two wheels weld themselves together, or jam up. Symptoms I noticed were linked to benefit 1 below. All the transmission gear rattle is heard/vibrated through the car. Noise and vibrations, really only when engine under load. I seized mine on my last lap of the day and did not notice it until I was pulling up the slope exiting the tunnel when leaving Goodwood - John M

found by John M on www.google.com search engine under DUAL MASS FLYWHEEL :- (haven't found out how to copy their pictures in yet - you'll have to look up Google! - Rob)

Double-mass flywheel Some slight irregularities in crankshaft rotation, caused by the ignition pulses, are impossible to avoid, particularly at low idling speeds. The double-mass flywheel prevents these from being transmitted as vibration from the engine to the gearbox and hence to the car's interior.
The double-mass flywheel absorbs the unavoidable residual vibration which the engine's firing pulses would otherwise transmit to the gearbox and driveline. Since 1987, Chevrolet, Ford and GMC diesel powered light duty trucks have been factory equipped with a dual mass design flywheel. Providing vibration dampening action superior to standard clutch disc dampening technology, this type of flywheel typically cannot be resurfaced, it must be replaced.
Why is the Dual Mass Flywheel needed? Transmissions in Chevrolet, Ford and GMC diesel powered light duty trucks have a heightened sensitivity to fluctuating torsional inputs. Result? A strong torsional resonance mode (vibration) that occurs within the normal driving range.
How does a Dual Mass Flywheel provide superior dampening? A Dual Mass Flywheel relocates the damper from the driven disc to the engine flywheel. This dampens engine torsional vibrations more than is possible with standard clutch disc dampening technology. The new resonance point is below the engine operating speed (idle), causing the flywheel to operate briefly at resonance during engine start-up and shut-down.
What are the Benefits of Dual Mass Flywheels? Eliminate excessive transmission gear rattle, making driving comfortable at any speed Reduce shift effort Increase fuel economy Borg-Warner brand Brute Power® Dual Mass Flywheels match original equipment in fit, function and construction. Just as in O.E. flywheels, Borg-Warner brand Brute Power® Dual Mass Flywheels utilize double row ball bearings, heavy-duty damper springs and virgin friction material. Only this type of construction offers the unique vibration dampening characteristics necessary for comfortable driving at any speed. To keep your customers' diesel trucks running smoothly, insist on Borg-Warner brand Brute Power® , O.E. match Dual Mass Flywheels.

It started as a noise which appeared to be coming from around the off-side rear wheel bearing area when taking a left hand corner/bend VERY HARD!!! and sounded just like a wheel bearing failing. Replacing the bearing did not help.
Then I started getting an occasional, but, horrific metallic rattling/graunching (very technical) noise (found to be coming from the bellhousing area once it was up on the ramp) on overrun & feathered throttle which progressed within less than 100 miles to an almost consistent metallic rattling/graunching noise.
Suffice to say, it was trailered down to Joe at La Rose Porsche near Sevenoaks (my favourite Independent) who reckoned it could be either the torque convertor or a cracked flexiplate (I thought it was called a flywheel!). After checking it was not a dodgy starter motor, Joe dropped the engine & gearbox but there were no cracks in the flexiplate so the torque convertor went away for cutting open. Apart from "a little bit of swarf" in the oil, nothing wrong was found & it was reassembled (welded back together) and returned to Joe. Joe started talking about stripping the box down which frightened the hell out of me as his "auto box expert" had never actually done a Tip before!! and even Porsche send them back to Germany to rebuild as no-one over here has the training. Porsche wanted £7,800 for a rebuilt unit so that wasn't the answer either!
He then wisely decided that although the flexiplate looked OK, it had to be worth trying a replacement one (£140) before any major gearbox surgery. The old one didn't appear to be warped until held against the new one!!! No more bearing noises (means I can thrash it round lefthand bends again) and no more horrible noises. So that was the problem all along; a warped (but not cracked) flexiplate - Dave B