Sounds like gas in the crop from sour crop or impacted crop. Aspirating the air from the crop with a needle would provide temporary relief from the gas but unless the impaction is relieved and thus the sour crop the gas will build back up as it did with your chicken. I have a chick at home that we are nursing for the same thing. Here's how I'm treating it based on my research: I first attempted to massage the swollen breast area (crop) gently from her head down her chest to attempt to move her crop contents on down her digestive system but it wasn't budging. I then purged it's crop/induced vomiting by gently massaging the bloated breast area up towards her head (the opposite direction as before.) I could hear the gas bubbling/escaping from her beak and shortly thereafter the contents of her crop was vomited. I let her catch her breath and clear her airway. Her crop felt better but still a little bloated so I repeated the process. After that her crop felt almost flat/normal. I then gave her approximately 2 ml red wine from a syringe (Yellowtail Shiraz is all I had on hand but cheaper stuff apparently works just as well.) She wasn't wild about the taste. From what I can tell, the wine acts as a laxative while neutralizing the pH in the crop/digestive system. She did act a little drunk/wobbly for an hour or so but then recovered just fine. I isolated her in a separate cage with a heat lamp and some water with 1 tsp apple cider vinegar to 1 quart water. This is supposed to also aid in killing the offensive bacteria that has fermented in her crop causing inflamation and gas. I isolated her so she wouldn't have access to food. When we first recognized she had a problem her stools were runny and foul smelling. Since the wine her stools have turned clear and watery. I considered the molasses and water mixture as a laxative but felt that the simple carbs would only encourage bad bacterial growth. I'm keeping her NPO (nothing per os/nothing by mouth) except the ACV water for 24 hours and then will start her with bread soaked in olive oil to lubricate her digestive system if her crop is not swollen/gassy in the morning . If she tolerates that I will offer mashed hard boiled egg and kefir and see how she does. If all looks well, I plan to put her back in with the rest in a day or so. She's not happy about being separated so I don't want to stress her anymore than I have to.
Here's how my chick presented:
1) bloated/swollen crop that persisted for a couple days (before we realized it was a problem. At first we thought it was her breast feathers coming in making the fluff stick out.)
2) soft, foul smelling feces
3) neck stretching and twitching/shaking motions of her head with her chin tucked
4) at first her energy level was normal then this morning prior to treatment she started to get a bit punky and was lying with her head tucked under her wing while the others were up and about.
3) she was eating and drinking and otherwise behaving normally
I hope this helps and your chicky does well.