I'm glad I didn't wait until it disappears itself, because it'll NEVER go. It turns out to be a head-snapped belt tensioner center bolt.
Initial symptom: metal clicking sound (bolt head jumping around), and then, squeaky sound (belt edge scratching), then LOUDER and LOUDER.
At first I thought it was just the belt, because it disappears after 1000 RPM. Only happen at idle or very low speed. Last night, took it apart, wholy sh!t, tensioner almost fell off.

So this morning, pick up a bolt from a dealer ($5), and install it on. Whalaa!!!!
Just wanna share with you guys, cuz the folks on this forum are the best!
Happy weekend!!!