Our conner does that when he pees. we took him to a very good glider vet. an examination showed NO signs of any problems. connor eats his OHPW and some happy glider pellets, but has never touched his fruits and veggies much in the 2 months we have had him.
so he eats and runs on wheel and vet could find no visual health issue, so he suggested connor makes that noise while peeing out of habit from maybe having a previous UTI. He said it wouldn't be good to load him up on antibiotics without more evidence of a problem.
If your gliders are together I would think if one is ill, the other one is probably picked it up by now.
Also we have a colony of 7, with one or two sometime bully gliders. If Mary gets in the face of Link, Link will make that noise LOAD AND LONG, he will go on for 3 to 5 seconds, telling mary to get out of his face. There has never been any damage from the bullying, just some chasing and lots of that NOISE.
I would see if there are any other glider experienced vets in your area. Our first experienced vet cost $100 and showed some lack of experience, not knowing how to even cut their nails. Our second vet has 2 experience glider vets and cost $46 a visit.
I would suggest asking if they can look into a UTI issue. Urinary tract infection..I think it means.