My '00 SE (with 60K on the odometer) has developed a very annoying exhaust rattle/high pitched vibration noise at around 1500-2000 RPM and *only* after the engine wams up and *only* while driving. First I checked all the heat shields and baffles, nothing loose, all tight and very little rust. I even took it in but they couldn't find anything. They suggested it may be coming from inside the cat which is scary. Anyone experience this?