The Rain sound has a weird "clicking" sound which sounds like the Menu-Click sound (only similar though), which appears about every 15-20 seconds, when it rains. Video link is here, sorry for the talking, but I felt it was necessary to point out when it happens; http://www.youtube.com/watch?v=B5gtmoTY1Pc
The sound appears like a variation of the Menu-clicking sounds that are used in the main menu or the Escape-Menu (Respawn, Abort, Save, Load, Configure, etc.), but slightly different. Also, it is not something in the game, as you can see I'm just laying in the open, not doing any special things, not pressing any buttons, just idling. It happens if in a vehicle (3rd person), and playing any unit and in any terrain setting.
It is definetly not the crickets or other animal sounds, as it is too consistent (happens always after a fixed amount of time). It is also not a character-equipment sound, as it is happening with no weapons and clothing on, inside a vehicle (3rd person), etc.