rattling from undercarriage, i believe it may be the catalytic converter?

This noise should be easy to diagnose. A bad catalytic converter should at least set a P0430 trouble code. Even with the engine cold if you crawl under the truck and hit the cat convt you will hear it rattle if it is breaking up internally, hit other areas of the exhaust and see if you have a broken baffle in another exhaust box or perhaps even a cracked or damaged heat shield. You may be on the right track ,look for a damaged exhaust or broken exhaust hanger. I would be surprised if you had a bad cat. convt. without a P0430 code being stored. A professional grade scan tool can display raw sensor data which when all looked at together may explain the idle problem. A professional scan tool may also have bidirectional control which can test the idle speed control to see it is operating correctly.

or the fuel filter directly in front of the tank has become unstrapped and is hanging and by the way needs to be changed every 30,000

Mine started rattling a bit and the Muffler shield on the bottom of muffler a weld dame loose. Crawl under the truck and bang on the muffler sheild and see if that makes the rattle sound.. I just bent it down a hair away from muffler and no more sounds