Strange resonance at 3-4k - coming from the Exhaust? Manifolds?

Hi All,

I have had this for some time now but the exhaust is producing a warbling noise when on part throttle between 3-4k when the engine is hot. It still pulls like a train but the exhaust is making a wowa wowa wowa noise like an oval resonance. Also notice the car is popping more on the downshift? Looked over the mounts and all seem solid. Had the car at wests on the ramps and no holes found? doesn't happen when static and engine is purring like a kitten? Any ideas? Could it be the manifolds?

How much are new ones?

K