Wheezing?

I've got my three girls in a large Martin's cage. Their substrate is Aspen/carefresh. They get no stress in their lives, other than being handled, which doesn't bother them a bit... But just tonight, I noticed they seemed to be "squeaking", but lower pitch. Upon listening closer, it sounds like wheezing!

2 of 3 of the girls are doing it. They were fine last night, and I didn't notice it this morning. What am I looking at here? They seem happy and active still... I'm dreading the worst here...