I have looked around the forums for this but let me give you the symptoms.

BMW X5 4.4i 2005.
Service Engine Soon light is on and its for code: P0420.

When the car is idling it sounds like rocks or marbles are falling into a tin can, and I have read that its the inside of the cat that is making that noise.

Sometimes the car shuts off when in park and idling but this is before the SES(CEL) appeared. It will idle search and stall out.

I was thinking maybe I should get a whole new exhaust system headers back, but I can't find any full systems.

I was thinking remove the cats and have the CEL coded out for the cats. Or I could replace the cats with BavAuto cats, for the X5 4.4i for $649.50.

What fixes have you done? whats a cheap route something that will keep me under $1,000?