You know it is easier to look stuff up then to guess about your local laws:
read section 3
http://apps.leg.wa.gov/RCW/default.aspx?cite=46.37.390
It appears that it is illegal to change you exhaust to an A/M exhuast in your state, if it is louder then stock, like it is here in KY. Again though it is up to the officer to cite you or not. I have received one such ticket here and it ended up being 'forgiven'/thrown out by the prosecutor.