My 91 had a very light and subtle but really irritating knocking noise at idle, kind of like a tapping on the firewall area.  Townsend mentions an additional fuel pressure damper to solve this, and it worked for me.  Well, the second one did.  Saab came out with an updated damper, but it didn't do a thing for me.  They then came up with an additional one that goes into the fuel line before it connects to the fuel rail; that one did the job.  See the Townsend site for additional info (link is on the Links & Info page).  E-mail me if you can't find out what you need.  Sorry I didn't document this better, but that was way back before this Web site was even a gleam in my eye.