Knocking from front left when going over speed bumps at slow speeds
 

My friend's car and mine (07 WRX wagons) both have a knocking sound coming from the front left when going over speed bumps or rough road surfaces at slow speeds. I have been told these are the coils of the spring touching. The car drives absolutely fine. Springs are the prodrive ones and the rears do have some plastic rubber around the coils but the fronts don't. Is this something to worry or not?
The problem happened a few thousand miles after fitting prodrive springs to the car.
In my car I had both front and rear drop links changed with whiteline ones when the springs were fitted.
Both cars have around 20k miles.
Thanks for the help :)

I had the same problem on my 54 blob. Had to remove then refit the front springs, the top spring seat wasn't aligned correctly. hope this helps.