Kinda hard to troubleshoot a rattle over IM but without a belt (which my first guess would have been water pump) I wonder if it could be exhaust related. Does it sound like metal-on-metal? Does it do it all the time? (Start cold and continue even warm). How many miles does it have? Is it RPM motivated? Is the idle smooth? Is it constant or sporadic? Can you get it on a lift and remove the bottom shied to check from underneath?
On the accelerator (which is basically the throttle position sensor) try a reset:
1) Turn key to ON position (not ACC), but DO NOT start engine.... now wait a few seconds for the dash lights to clear
2) Slowly & steadily press accelerator pedal all the way to the floor and then slowly release at same rate.
NOTE: This step must be done ONE TIME only to ensure accelerator pedal position sensor calibration has been
learned by the ECM
3) Leave ignition switch ON for a minimum of 10 seconds. This will allow the PCM to learn the new electrical parameters
There is another method if that doesn't work:
1) Turn the key to the on position, the position just before the starter turns over.
2) Press the gas pedal firmly to the floor with the key in the "on" position.
3) Hold the pedal to the floor for five seconds, then turn the key back to the "off" position (don't remove the key), then release the gas pedal.
I don't know if there's a way to test the TPM, I would assume it's a glorified variable resistor but certainly could be more. It could be a dirty throttle body. Unfortunately, to replace the TPS is either $100 or $300 depending on version. The best troubleshooting step is to replace with known-good. If you get one and can return if it doesn't fix; I would.
Here's a link to diagnose: Chrysler 300: Bad Throttle Body → Symptoms + Diagnosis | Drivetrain Resource
I listened to the video and all I could hear was 'awesome' but no rattle. How many miles are on it? I assume the oil level is full. Does the transmission behave properly? What kind of air filter is in it? (not starving for airflow)
Mine is close at 148.000 but has a lifetime powertrain warranty. I have not found any info on the bearings (are they sleeve or roller) but it may be time to get the dealer to diagnose. At around $140 a reliable conclusion can actually save money.
Any progress on the idle lag?
Yea, this is actually what I have been thinking. The gen3 hemi is a roller bearing I believe, not looking forward to pulling the heads off if I am honest. Was hoping I could pinpoint it without doing so but I'm running out of ideas haha