What is causing this to happen and what is it?

Scan test 'Antilock Brake System' is required! .... ABS also has control over ESC system ... SCAN TEST ABS!

Hello! Your vehicle is equipped with Rear Anti-lock brakes. As the other person stated, the ESC system shares the same components. There is a hydraulic control unit that could be making noise, but unknown -- the noise may be entirely unrelated to the ABS system and could be a steering issue. Nonetheless, all you can do is run a diagnostic test of the ABS system to find out what is failing, then pursue this for repairs. If you'd like to find a repair facility near you that can help, click this link: http://repairpal.com/kia-within-10-miles-of-minneapolis-minnesota-55415 Good luck!