I just thought that I'd add to this as I've had the same problem in the last couple of weeks, a knock that sounds when I go over bumps or on steering.
It turned out to be the big nut holding the damper in the strut (not the one at the centre of the strut top but the one under the bump stop at the centre of the spring).
I replaced this with the propper one as it had one with too coarse a thread on it and the insert was pretty much rattling around in the tube because it wouldn't screw in far enough.
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