Definately sounds like a harmonic balancer problem to me. Mine went out on Friday. It happens all of a sudden and makes a huge clattering noise at idle then goes away when you rev it up. Its real hard to tell where its coming from, and of course I panicked and thought supercharger bearings or coupler. (Mine is a 95 SSEi with 170,000km/100,000mi. by the way)
Thanks to all the great posts and links here, I was able to properly troubleshoot it before I got carried away and spent all kinds of money.
To eliminate the accessories, take the belts off and run the engine briefly. I actually took the inside belt off first because I wanted to check the supercharger. If you do it this way though, be very careful to leave the belt so it doesn't get tangled up in the other belt. Remember this belt drives the water pump, so only run the engine briefly to avoid overheating. If the noise is still there, like mine was you have eliminated the supercharger, air conditioner and water pump as problems.
At this point you could swap belts and eliminate the alternator and power steering pump.
In my case I took off the second belt as well and of course the noise was gone when I ran the engine. Alternator and PS pump spun freely by hand so I was pretty sure it was not them.
Finally I grabbed the crank pulley/harmonic balancer and tried turning it. There should be no movement there...unless you are strong enough to turn the whole engine. Mine however moved slightly and then stopped. When it stopped it made a metallic ping with a similar tone to what I hear when the engine is running....AHA!!
Anyway, I'm now shopping for a new harmonic balancer and think I will get a local shop to put it on. Apparently its not a huge job so the labor shouldn't be too serious.
Hope this all helps. I feel kinda sick when I read about all the folks who spent hundreds on other parts to fix this problem. I'm not being critical. Without this club I probably would have done the same. Hope this keeps someone from going the wrong route on what appears to be a very common problem on these cars.