10 week old Australorp constantly "swallowing" possible crop issue?

We received five 8 week old chicks about two weeks ago and shortly after noticed the Australorp making what I would describe as "swallowing" gestures. I didnt see the behavior in any of the other chickens nor have I since. Its been two weeks since I first noticed and she is still making the gestures. Ive felt her crop both in the morning and late afternoon. It seems to be empty in the morning and very full in the PM. Larger than the other girls but then again she has a larger appetite than the rest. She eats and drinks and seems healthy. Her droppings are normal. I held her and massaged her crop a bit today and she does the swallowing gesture and coughed a few times and I noticed a bad smell (wondering about the possibility of sour crop?). Like I said its been about two weeks that ive noticed this behavior but she otherwise seems healthy. Any advice would be appreciated.