2004 Chevrolet Impala Sounds rough when running

Car is making a sputtering noise underneath, and there's a funny smell outside and inside the car. Car is running a little sluggish when coming up to speed. A garage told me that it might be the converter and to bring it in to check it out. He said that if it is the converter, it could cost $400 to fix - but can't give me an estimate until he checks it out. So far, he just listened to it. He also said he smells something. Said the converter could be clogged.