I had this from about 18 weeks but worked out it was actually my pelvic bones grinding together- got diagnosed with SPD 2 weeks later! Ouch. Different thing! I asked my friend about it (she just had her baby) and she was told by the midwife it's the baby's joints clicking, like when you stretch an elbow or something, as the relaxin hormone affects the baby's joints as well as yours and makes them looser and more prone to click. Maybe.