Unknown death & chicken making a croaking noise

Until yesterday I had a flock of very happy healthy chickens. Yesterday morning Betty ( a 1 year old Warren) seemed poorly. She wasn't interested in food and took herself off into a corner of the garden away from the rest. She took herself to bed last night and was OK when I closed the door last night. This morning I was shocked to find her dead in the nest box. There was no bleeding or injury, I checked her over and couldn't see anything wrong with her. She hadn't been picked on by the other birds and I don't have a case of mites or anything like that and her comb looked lovely and red - normal weight and healthy looking bird. I haven't noticed any unusual poop, and she wasn't off colour before yesterday.

Today I have noticed that Annie (also a 1 year old Warren) has been making an almost growling sound, a bit like a frog. She has been eating and drinking and has had cuddles and I have checked her over and I can't see anything wrong with her.

I have thoroughly disinfected their coop (using animal friendly disinfectant) and I have redusted it for mites and changed their straw bedding. I have also been and bought a tonic for them (in pellet form).

Has anyone come across this noise before?
I haven't noticed it before and am starting to become concerned. (btw I didn't notice a noise from Betty).
Any advice would be much appreciated.
Many thanks.

 

Hi,

Annie is making the noise when she is breathing in and out.
I have checked her and spent time with her this morning. She has eaten, although not as enthusiastically as she would do normally, and she has drunk a little. Her vents are clear and her eyes are bright.

Many thanks for your help.