. Could be a plugged catalytic converter (which will cost some cash). Or a simple hanger let loose. Or the ox sensor rusted out. Get it checked out, ask for them to show it to you while it's on the hoist and either swap it yourself later or have them do it while the car is already up in the air. Most often, chassis and muffler problems ... just pay the guy at the garage with the tools to do the work. Crawling around under the car bending rusty nuts are no fun for the shade tree mechanic. .