Ball joints are the bearings that connect the control arm wishbones to the steering knuckles, there placed on either end of the drop links. These act as a pivot and allow them to move together creating a smooth motion in the suspension. When they're worn the ball joints make a faint clunking noise very similar to worn suspension drop links
or the tie rod ends
so it's important you validate the issue. Another way of diagnosing bad ball joints is stiff steering or excessive play in the ball joint, if this is the case you might be better off replacing the whole wishbone
instead. If you find there's an issue with your ball joints it's best that you buy replacement ball joints as soon as possible.