Try listening with your ear near the catalytic converter. On every vehicle I have owned, the converter made a noise like boiling water. It is just the catalyst cooling off and should not be a problem. I also have a 2.5l in my XJ and the exhaust pipe makes a boiling type sound as it cools off. Just the metal expanding and contracting with the temp change.
84 Wagoneer, 207k miles with 4 angry mice under the hood. Uniden CB, Kenwood 2-meter. Tow Hooks, front skid plate. Not pretty, but it runs well, goes where I want it and it's paid for.