var camera_bob = 0;
...
in your player action:

camera_bob += 10*(key_w || key_s || key_a || key_d)*time_step; // assuming you have WASD movement
camera_bob %= 360;
vec_set(camera.x,player.x);
camera.z = player.z+player.max_z*0.75+10*sinv(camera_bob); // you can try different values here
...