Sour crop can sometimes be a secondary issue from antibiotic use, a fungal or bacterial infection, late impacted crop, and many hens with reproductive infection or salpingitis can get crop stasis which leads to crop problems. Gurgly sounds can come from crop contents coming back up into the...
Try using some plain Greek yogurt or probiotics to get her beneficial gut bacteria in order. A small cup of Fage brand would last all week. What does her crop feel like this am? Here is a good article on crop disorders by Azygous...