it doesnt have to rattle bro! a lot of times, when u get ur engine get a fuel system service, the engine is not able to burn all the cleaner. That cleaner goes to ur cats get wet(hahahaha wet cat! hahahahah) it plugs, not only from the cleaner but also from any unburned fuel that a faulty spark plug, wire or leaky injector let thru. get a set of straight pipes or open the exhaust system drive around
and see if it runs better.