Not sure why you thought parking with the brakes off was a good idea or that they would not engage when the air pressure dropped. Sounds like you need to study up on air brake operations. Since it's a loud noise you should be able to find where it's coming from. I would try engage the parking brake several times.
PS: Welcome to the forums.
This post is my opinion (free advice). It is not intended to influence anyone's judgment nor do I advocate anyone do what I propose.
Sold 04 Dynasty to our son after 14 great years.
Upgraded with a 08 HR Navigator 45’...