Well I've never heard of this but energy does pass through crystals so audible noise may well be possible. ON the other hand, is it possible that you thought you heard it but felt it instead, that also may be a possibility? I have felt crystals but never actually heard sound from one, though as I just said that I started thinking about radios which use crystals to decode radio waves, so I guess it is possible.
When was the last time your cleansed the crystal? If not recently I would recommend doing so because in ,y experience when a crystal hums, whistles, etc. it's due to it having too much energy in it & if it doesn't get cleansed soon it will stop working & end up cracking & sometimes bursting/shattering.