What is the cause of a whistle sound on left wheel It's intermittent and as soon as I hit the brake it disappear. It normally happens when I driving on speed of 31 mph. Is it perhaps wheel bearing?

It is a "chirper spring" that is warning you that you have very little brake pad left.

------------------------------------------------------------------------------------------------------- Thanks for the answer, but I have already replaced the brake pads and brake disc. Before, I replaced that stuff, the sound was not there.