thanks for your replies..........
actully this sound is coming before rains......thats y i dont think so its because of rains.....
and i shoed to my mcaninc he also couldn't understand where from sound is coming he checed the belt and said belt is ok...
i also applied WD40 on belt and still having that problem. yesterday i saw one vitz in red color 1999 he was also having same problem.