That rattle only at idle could also be your harmonic balancer. Although a coupler tends to be a rattle, and the HB tends to be a clatter. Hard to describe.
You'll have to remove the SC entirely from the car for a Series 1 coupler change. You can also rebuild your ND yourself if you have experience replacing bearings. Otherwise, a rebuilt ND from www.canadiansupercharger.com
may be in order. With the miles on your car, it'* a safe bet.
After that, the first thing you need to do is get a scantool. You have to know if you have KR before and after a pulley change. You also need to verify with a gauge that your fuel pressure is up to par and will support a smaller pulley. In addition to this, you MUST verify that your exhaust manifolds don't have any cracks and aren't leaking. From there, you should be ok. On a Series 1 L67 with a smaller pulley (one step down, for you would be the stock 92/93 pulley or a 2.5" from www.pulleyboys.com
), I'd strongly suggest a high-flow catalytic converter, particularly with your high miles.
Don't forget the basic tuneup items. Including an O2 sensor if it'* been in there for 50-60k miles. It needs to be in top form.