I'd start with the exhaust, sure sounds like a leak to me. Either loose or cracked manifold. Any loss in performance or funny pulses? Find a good hill, come to a stop, and slowly add throttle, all you're doing is adding a load to the engine slowly, see if you get any hiccups. Hiccups may indicate a leak and caused by bad readings being sent to the ECU by the 02 sensor. Next step would be using a mechanics stethascope, less then $10 at an auto parts store and easy to use, start at the point of noise and narrow it down to where it's the loudest. If it aint the exhaust I would guess it's the valve train, maybe a collapsed valve spring, if that's the case get it fixed quick, you really do not want to drop a valve.
Hope this helps,
Chet