STRUT ROD BUSHINGS
Bad strut rod bushings will cause a knock when backing and then going forward.
For example; if you put the car into reverse to back out of a driveway you might hear a knock. You then put the car into drive and start to go forward and you get another knock. This is bad strut rod bushings. Also, if you have a bad power steering pump leak then check your left strut rod hushing because the pump is directly above that bushing and it leaks onto it causing it to fail. Power steering Said is very bad on rubber.