C-2 Stereoset : One mic in the pair came defective

Both microphones in the set worked well in the beginning. Then I put them back into the box and only used them again a few months later. It appeared by then that one of the microphones produced a noticeable noise like there was a sand constantly moving inside the capsule. I switched the capsules and confirmed that the problem was indeed inside the capsule. I was able to make the noise disappear by giving the capsule a light kick with my nail, and the microphone worked fine for that day, but the problem reappeared a few days later. I "fixed" it again by kicking the capsule. I don't know for how long this kind of "fix" is going to work, and I don't like having this unreliable stuff as part of my setup.