This is not my link, however, is one of the few videos I've found of the noise. Is this noise actually called whistling? I can't seem to find any information on it? This is the noise I'm greeted with by one of my gliders. It sounds happy but I'm more just wondering if it's actually a good sound. It seems the only sounds people really document are hissing, chatter, crabbing, and purring. I feel like there are so many more!

youtu.be/lmyceuelQJg

I've never heard that one before. I wonder if it could equate to a little bit of wheezing. Their little lungs going so fast breathing in and out. I'm not sure that this is an actual verbalization. Perhaps it is like the popping I have heard. It's all extremely faint.

My girls make this same sound! I notice it most during playtime, and I've always kind of assumed it was a happy sound. My girls certainly seem happy when they make it. I don't know for sure though. You're right that only a few sugar glider sounds seem to be documented/explained.

My gliders make this noise, especiallllly when I have to take just one of them out for a bit and they are reunited :)

I've not heard that sound before but it does sound like a happy one.

there is a few spots in the recording that sounds like a very fast scratching.
at 11 seconds and 17 seconds. grooming sound.
if they made this sound in pouch, I would think they are awake and grooming.

ours also can make that type of noise while eating live mealworms.