I believe the "twang" being heard is caused when the spring is a bit too long(and powerful) (basically agreeing with iowsniper) My theory behind this is simple, because the spring is longer than the cylinder and because it is so powerful the spring, when fired, has room to bend in between the end of the spring guide and back of the piston which will cause each coil of the spring to make a noise and because a good number of coils will do this a twangy sound will be produced. (this sound could also be coming from the spring rubbing/popping off of other parts)
Its kinda hard to explain but what you can do to fix it is:
*get a spring with a thicker wire making up the coils
*check and make sure the spring fits 'perfectly' around the spring guide
*make sure your spring guide does not wobble around in your cylinder
*see if you can mod your piston if it is wobbling in your cylinder
This happened to me with and OLD bar10 of mine when I both stretched the spring(dumb kid back in the day
) and when I installed and upgraded spring. I am not sure if this will work in the CA M24 but the ideas should be the same(and the general function of the two rifles is identical) Also, if your piston slides perfectly flush in your cylinder it should increase the lifespan of your sears.
I would advise that you shim your trigger box if possible an try to increase the spring tension on each spring within the trigger box, that might help a bit.