I have a recently acquired 99 GTP. Supercharger sounds fine at idle - not rattles -though it does seem to get quieter at 30+. It's been fine driving at low speed and highway speeds. But on a certain trip with some hills at highway speeds when it downshifts to climb a hill and the boost jumps to 80-100% the car vibrates and there's a unhealthy rattle/vibration that sounds like it's coming from the motor. That's really the only time I've seen boost that high so I can't say if it would happen under other driving conditions.
I did change the supercharger oil, was low and pretty dark. Check a few plugs and looked ok. Scanner shows no codes. 206K on motor, 70K on rebuilt transmission. Any thoughts, hopefully it's not destroying itself