Thanks Bro Humms for the fr and Bro Alantiong for the contact to Titanshock. I finally replaced my absorbers yesterday and true enough the knocking sound is gone.
I was surprise to see that the original BMW absorber is so soft, you can apply force with a single hand to compress the absorber. It's that soft, unbelievable.
After installing the absorber, I can feel the immediate improvement in terms of the car handling. The steering feel is excellent, I took it all over the town for a spin and the adrenaline feel of driving a BMW sport mode came back to life. I almost forgotten why I bought a BMW in the first place.
Kudos to Titanshock on the excellent buatan Malaysia product and great service. We'll done !!