My Evo9's previous owner replced the stock suspenion with Tein MonoFlex.
Started yesterday when I go over numps at 40-50km, and when I go around fast corners, the left side front & rear suspension will generate knocking sound, it could be pretty loud too.

Would like to know if anything needs replacing or it's a characteristic noise of Tein MonoFlex?

 

No point in second guessing suspension issues, might as well get a pro to look at it. Knocking could be the camber top or even a blown strut in the coilover.

 

Check to see if the center collar of the coilovers are sufficiently tightened.

 

^^ I would suggest to try a little more preload on the spring.