I suspect your car is put together much the same as mine, and I would have a look at your swaybar and outer bushings. Frequently the swaybar itself wears in the area that constantly slides inside the outer bushing. On smaller bumps at that speed, the bushing play will cause a prominent knocking as the sway bar moves up and down with the suspension. All this assumes you've already ruled out the usual suspects (tie rod ends, ball joints).