Two years ago i had an avulsion fracture in my hip, no surgery required. but when i lay down/ stand up i feel something 'snap' in my hip. any idea?

Could be scar tissue: Could be scar tissue or a tendon snapping over the hip. Need to check it out. See an orthopod-need xray, possibly an mri.

Snapping hip: Snapping hip can be due to tendons in front, or along the side of the hip. Your avulsion fracture may have contributed to this but without ortho evaluation it is difficult to tell. Typically painless snapping is not a problem and stretching the muscles around the hip can help with most snapping.