Resonance usually occurs from a long tube vibrating, and given that the "flap" is actually inside the cast iron manifold itself ? Unless your use of the term resonance is different than mine.
You should be able to move the flap by hand to see if it is stuck in the closed position. you should be able to rotate the the big round counter weight with your bare hand and the coil spring will reset it back in place assuming the coil spring is intact and loaded on the pins right. The weight should rotate a little more than a 1/4 turn If I remember right. sometimes the stick and if you use some PB blaster it can free it up. (obviously only do this when the car/engine is not hot
)
If you really wanted to, you can pull the exh manifold, remove the flap, and then weld up the axle pin holes.
