well, ok then...look at the pic:
thats one whole brake with the caliper (the thing with scion written on) moved out of the way, the other red part is the caliper bracket, its not the one in the crosstrek but the concept is the same.
you can make out one of the pads (that rusty looking thing in between the caliper bracket)
as you can see the pad has two ears that go into a groove of the bracket, thats because the pads need to slide in to grab the rotor and stop the wheel from turning and slide out, away from the rotor, to let the wheel go.
If you look carefully between the red part and the pad, where the groove is, theres another small piece of metal, its an abutment (or antirattle) clip, heres a closeup of it:
Theres one on the top and one on the bottom, heres a pic of the pad ear and the clip alone:
when you buy the pads there should be the abutment hardware too, a set (for two tires so 4 pads and 4 clips) looks like this:
Sometimes there is some play between the clips and the pad (various reasons for that or the clips get forgotten) and the pad moves a bit up and down and so it slaps onto the braket making noise when it grabs the rotor.
That can be the click clack. It can be resolved using new abutment clips and some of them are even coated to quiet down noise.
Another source of the noise can be another couple components: the slide pins
There are two of them one on top and one on the bottom, the caliper attaches to them, you dont immediately see them because they are behind the brake, on the car side.
The slide pins also slide in and out, thats what allows the caliper to move so that it can squeeze the pads to brake and then let them go.
If there is play at the pins then again you have movement and that same slapping noise. If one changes the pins the noise goes away.
If with the car raised up and the tire off and brakes on (somebody helps you or using a stick) you grab the rotor and turn it you can spot where the play is. Thats what you want the mechanic to show you (or if you check by yourself)
Personally the only cars where I found that play as "normal" is in race cars, the crosstrek isnt a race car. Out of the dealer lot the brakes dont slap around, they shouldnt later on either.