At Muffler Masters this past week a wise technician enlightened me wrt to the strange vibration/noise I was experiencing both at idle and at speed (1900-2100rpm). I thought initially it was the original catalytic converter coming apart both inside and outside; there were signs of rust perforation etc. Once the new free flow converter was installed we could still replicate the noise by banging on the front Y-pipe. The technician remembered that GM at one time installed dual-piped exhaust pipes (one inside the other). The problem with a pipe inside a pipe is that the spot welding gives away with the inner pipe and it begins to rattle at varying rpm including at idle on some cars. The quick fix was to blast some holes thru the outer Y-pipe and re-weld the inner pipe to the outer. The car runs a whole lot quieter now with no rattles. For those experiencing this problem you can easily bang on the front Y-pipe and if it is shot you will hear the rattle. The inside pipe seems to only run the horizontal length of the Y-pipe and not up to the headers. It is a very strange set-up to say the least. Hope this helps some older C-4 owners!