Don’t be afraid to take it apart. By a rattle, do you mean something rolling around when you shake the melodica? It sounds like the spit valve mechanism has come apart or broken. If it has just come apart, for example if the spring has come off, it should be easy to put it back together. Just be sure to use a proper size screwdriver so the screw heads don’t get stripped. I thought you were already upgrading to a P-37D.