Its only a knock when the brakes are applied, and then you'd feel the pressure of brakes being applied. Its irritates.
I asked the MB dealer and he thinks it could be the case of either:
1. a dirty ABS sensor, which needs to be cleaned; or
2. faulty brake relay.
I notice this morning that the knock was absent (a wet morning). So I conclude that it comes up only when its hot.
How will the strut rod mount or wheel end mount create this knock?
Thanks for the comments.