20R sounds like a lawn mower

The 20R engine in my 1980 4x4 sounds like a lawn mower when I give it gas. I've listened to the engine under the hood when idling and I hear a hiss onces every 3-5 seconds towards the back of the engine. This lawn mower noise make the engine loud. Any thoughts on this?
Thanks,
Mike

Most likely a hole in the exhaust system somewhere between the exhaust maniflod and the muffler.