I fixed it eventually!
anyone got this rattling engine, just try what I was done, the tensioner has a recoil spring inside, keep a strength on a helicoid protrudent(I do'nt know what call that), the recoil spring was too weak already, I pull up one side of spring-end, give it 3 turn then set back, the sound was gone, I try the bike for several days and never heard it again,
just for reference of everyone.
Landy