Probably an actual exhaust leak. Get under there and look, listen, and feel for an exhaust leak. If you run your hand close to the pipes without touching them you should be able to feel an leaks.
Generally leaks would be at points of attachment. A loosed bolt at the mid pipe or a bad gasket at the midpipe/ header connection. Just hope it isn't an actual header leak.
Life is too short to do ing exhaust work take it to a muffler shop. Those guys don't charge and are masters with the taps, ez outs, torches, and other forms of seized bolt torture.