I have noticed lately that my car is making an abnormal sound from just behind the engine, but almost under that car. If I tap the throttle slightly and let off right away to keep the rev under 1000, it makes a somewhat "purring" sound. It sounds stronger on the driver side than passenger. It sounds almost as if there is a small hole in the exhaust letting air putter out. Again, noticeable at low rev.

Has anyone encountered something like this? If so, what was the fix? Could this be an early weld issue forming?