These shields or guards are fitted around the front hub, they are listed as stone guards, but I think they are also heat shield to protect the hub bearing, lower ball joint and the track rod end.
The shields are made from a light alloy and due to the effect of differing metals (the hub is steel) they rot around the mounting bolts until they come loose and start to rattle
I have seen it stated on other sites that the solution is to simply cut them off anf throw them away....well I disagree!
In the picture to the left you can see how the "stone guard" is shaped to follow the contours of the track rod end and also extends around the lower ball joint and tension strut bush.
I believe this is to protect it from the heat coming off the disc
In the picture on the right you can see the rubber boot of a track rod end that has become dry and brittle on the side that was facing the disc
This was taken from a car which had its the "stone guard" removed
Here is a shot of how the shield corrodes around the mounting bolts
The fix is cheap and simple, just remove the three mounting bolts and add six large zink plated washers with a little copper grease added to reduce the corrosion.
To do this, first remove the brake caliper and the grip washer fitted to one of the wheel studs, this will alllow you pull the brake disc away from the shield so you can get at the 3 mounting bolts Now remove the bolts and add two of the large washers (I got mine from B&Q) to each bolt, one at the back of the guard and one at the front
This should give you years of trouble free service AND protect your ball joints!!