I have a 98 Intrepid. When I am coasting or deaccalreating I can hear arcing from my (coils, spark plug, ? ). How can I find the problem?

Is this radio related or other (AUX/CD)? Do you have an aftermarket radio or AMP installed (I assume you do)? Normally engine noise is picked up due to improper grounding of an aftermarket system or inappropriate shielding of AUX wires.

Everything related to radio and speakers is factory. I'm thinking it's a bad spark plug or coil?

I'd try plug wires first.