7 week old chicks have very faint crackling sounds

[Theck1722]

I have 5 bantam chicks that I am/was so excited to finally get.
For 3-4 weeks now I’ve heard very faint popping/crackling sounds when I put my ear to their backs. You can hardly hear it without putting your ear up to them. No mouth breathing, all are eating, drinking, and active. Since ive heard it for weeks and they haven’t gotten sicker and are still thriving could it possibly be normal sounds?
My main concern is putting them with my other birds that we love.

 

[Theck1722]

I have 5 bantam chicks that I am/was so excited to finally get.
For 3-4 weeks now I’ve heard very faint popping/crackling sounds when I put my ear to their backs. You can hardly hear it without putting your ear up to them. No mouth breathing, all are eating, drinking, and active. Since ive heard it for weeks and they haven’t gotten sicker and are still thriving could it possibly be normal sounds?
My main concern is putting them with my other birds that we love.

Edit to say that 2 of them are growing very slowly and just now getting feathers