Experiencing popping / cracking sensation (no pain) in lower back when performing leg extensions or leg raises. Is this due to tightness in the hip area / hip flexors ?

Could well be. If you roll your thigh/foot in or out and it changes then it's most likely to be something snapping over something in the front (hip flexorish). If the thigh/foot doesn't alter then it may be  more joint related. Does stretching change it? Does ramping up your gluts change it? Either way if it's painless, crack on and don't even think it's doing harm! The Guru