i have a random tech high flow cat and mine doesnt rattle one bit....i have no complaints except my exhaust velocity is lower than it should because of the resistance loss by installing the cat......and according to random tech their cats will pass smog in cali if you bought yours from a authorized dealer you will get paperwork that has the carb exemption info in it....

and the check engine light coming on is the ECU sensing the resistance change in the exhaust and that is what is causing the check engine light...its nothing to worrk about....my car throws it about every 300 miles or so....just pull the clock radio fuse 7.5
and it will reset the ECU