I own a 2005 cadillac sts w/ 3.6 engine 100k miles. I recently heard a rattling sound that was similar to the ball shaking in a can of spray paint. The noise increases and decreases with increasing or decreasing gas to the engine, respectively. The noise seemed to orginate from the engine, but it also could have come from the center of the car, It was difficult to tell. The odd thing is even with the noise the car is running fine, except you can't use the remote starter with this issue. I called a friend and he said it could be a sticky lifter, so I changed the oil and hoped it would go away like he said. It did go away for a while then the rattling return. I drove it around town for a while thinking that maybe the oil needed to circulate through the engine.
The rattling actually stopped completely, then the Check engine light came on. I took it to Auto zone to get the code read, and it said Catalytic Converter below threshold (bank 1). I was told before I change
the Cat. Converter ensure the O2 sensors are working there could be up to 4 of them because I have dual exhaust, one before and one after the cat. I did think I would have additional codes if the O2 sensors were bad, but they probably need to replaced because of the mileage on the car.

That leaves me with the following issues:

1) I only have one Catalytic converter, and I have no idea where the O2 sensors are located. It didn't see them anywhere on the exhaust pipe.

2) Where can I find a Cat. converter with dual inlet and outlet ports? Every site I've been to has single ports.

3) What else could be the root cause of this issue? Bad fuel, etc... I really want to fix the root cause of the problem to prevent a repeat of this.

Any help or lessons learned on this issue would be a big help.