Dont mess around with exhaust gasses, (carbon monoxide), if you still hear what sounds like an exhaust leak, The description: "faint sound of a plastic bag caught on something and flapping under acceleration"
is a sure sign of an exhaust leak, so keep looking for it, and find it.
If must be noted that some exhaust leaks only happen when the exhaust system is cold, and as the gaskets, manifolds, and exhaust pipes warm up to operating temperature,
the exhaust leak closes, and less sound is heard. Sometimes the opposite applies, the leak is not heard when the engine is cold, but as the engine warms up, the leak becomes larger
allowing more gasses to pass through the gap making a rapid "tick,tick,tick" type noise under acceleration. But of course we all know this, and it must be taken into consideration when
an inspection is performed. However, some custom exhaust systems with headers allow more sound to pass through the thin wall piping, sounds that could be mistake for
an exhaust leak.

If you can hear it, then it is a fairly large exhaust leak.