Just take the resonator all the way off and inspect it. It could contain the carcass of some rodent that picked the wrong place to build a nest. Drive the car without the box and see how it sounds.
I should probably do that first. It sounds kind of gross if there used to be something alive in there.
My wife thinks that the engine is dying, but just at idle I could hear the noise coming straight from the resonator. I still don't know exactly what it does. I know that it's the air intake, but I've had other cars where the resonator (which I still don't quite understand) was built into the air filter box/case.
OK I looked up what it does. I don't know much about pressure wave harmonics, but apparently it's needed for performance reasons.