I get this at low speeds too. I've got Apexi 60mm lowering springs that I've lopped a coil off on each corner, so it's like 90mm lower I think? But I get a creaking/knocking noise from the front of the car at low speeds as well, particularly when parking or just turning, similar to yours.
Basically, because the suspension struts are measured out to take a certain length of spring as standard, when you put lowering springs on, they won't fit quite snuggly in the struts. This is especially evident where you've got the slight lip on the top and bottom plates - the standard spring sits in snuggly without moving, but with lowering springs in, they move around slightly becuase they're too short to reach the lips. This is where the knocking noise comes from. And my car recently came back from a service and MOT (with new front discs and calipers, so would have had a good view of the suspension to notice if anything was broken), so it couldn't possibly be anything else.