I was just sitting in park and started hearing a clicking/rattling noise. Could the rear catalytic converters be rattling and it sound like it is in the center of car near drive shaft & transmission go together? I have come to believe it may not only be loud in park and neutral but in like first gear before shifting. Or could something in the differential sound like its in middle of the car? I am lost!

 

But if it's noisy in park, it rules out mostly the driveshaft. So... this could be either the transmission mounts or the catalytic converter.

I would check to make sure the drivetrain mounts are good (motor mounts and transmission mounts and even the exhaust hangers.

differential noise tend to be in the rear of the car, usually whining noise. Check the mounts first then the converter itself if the mounts are good.