Is it a ticking/ rattling that is worse after the car heats up? I am getting mine replaced today for free (catalytic converters were $3295 alone) because we have an 8 year/ 80,000 mile federal warranty for them. I have 78,000 miles on mine and it has done it off and on since I purchased it with 58,000 miles. My mechanic told me that they had a "brick" inside that comes loose and I have heard there are welds that can break too but all I know is that it is getting fixed for free. It did not throw any codes but I finally got a dealership to admit that it was the problem. I also think that I have a minor engine noise but we will see if it disappears completely after these are put on. I do not believe that the other noise is/are the injectors as I have a street biike and know very well what loud injectors sound like. Anyway, if it is a rattling after heating up my guess would be the catalytic converters.