Hissing

I have an '02 Dakota Quad cab with the 4.7 V8 with 63,000 miles. Recently bought it and just after we got it, it has made a hissing noise when it runs. Something like a hose with a hole in it. Everything works fine, AC, heat, no oil pressure problems, transmission is ok, 4x4 works. After the truck is turned off, it hisses for about 3 seconds and then fades off. Just curious. I doubt it's any condensation dripping on the exhaust because it happens even though I don't use the AC. A Magnaflow system came with the truck, would that have anything to do with it? Maybe a bad seal or something like that? Not much of a mechanic, but I can at least troubleshoot things if I know what I'm looking for first. I'd appreciate any help offered! Thanks again!