The rattling could be the cat. What is your fluid level like? Check the fluid with the engine hot and running, and preferably after driving it a bit. Does the fluid smell burnt or have a brownish color and not red color?
You may just need a good flush and fill. Its hard to find anyone that will do that anymore. The 4L60E transmissions torque converter carries fluid in it to operate, and the only way to really get it all out is to have it flushed. Though, it could just be going bad. I dont know much more than that about transmissions.
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Exploration Machine- 02' CC V8 swap. 300K+
Portable Roller Coaster- 07' MKV VW GTI
Its another Jeep- 98 TJ