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We have a 93 Park Avenue Ultra with a rattle that seems to be coming from the s/c pulley or the tensioner next to it.
I removed the belt from both and the noise disappeared. The s/c pulley seems to turn easily enough by hand.
The rattle goes away at 1000 RPM. How can I determine what is causing the noise?

Put on one belt at a time and see how that effects the rattle. Is there any slop (rotational or side/side or up/down) in the SC pulley?

The s/c has new bearings and a rebuild about 8K ago, with no slop that I can notice. I can't really remove the belt from one or the other as it's too long to do that. I may take my drill and a lawn mower belt and spin them one at a time to see if either makes the noise.

A long screwdriver or mechanics stethescope would help you out a lot.

I meant put on the SC belt by itself, then the other by itself to see if it isolated the noise to one set of pullies. If it's present with both, you'd probably narrow it down to the harmonic balancer. The rubber in them can crack over time and it'll rattle like sunovagun, and be difficult to tell just by listening.

The noise is only made when both belts are on. If I remove the belt that runs the s/c it goes away.
When I put the screwdriver to it I can't tell if the sound is coming from the s/c or the tensioner.
I am leaning torward the tensioner because it shakes more than the s/c pulley, but then again, it's supposed to absorb vibration, right?

A tensioner should not vibrate up and down at idle. If it dances, replace it.

Replaced tensioner, noise still is there. Fluid on s/c is up to the threads, although it is dark. The s/c pulley feels smooth and I can hear the air moving when I spin the pulley.

Sound like it could be the coupler in the supercharger, hand rotate the pulley back and forth rapidly and see if it starts to make the noise?

J Wikoff wrote:I meant put on the SC belt by itself, then the other by itself to see if it isolated the noise to one set of pullies. If it's present with both, you'd probably narrow it down to the harmonic balancer. The rubber in them can crack over time and it'll rattle like sunovagun, and be difficult to tell just by listening.

Winner! The harmonic balancer is being replaced as I write this, I didn't think $300 for labor & parts was too bad.
The garage owner called it when I pulled in to his driveway, he asked if the harmonic balancer made that rattle all the time, I said no, only under 1000 RPMs.

Hm. Mine did this when I first got it. Changed S/C oil and good to go! But good to know that it could be the harmonic balancer if it ever happens again!