Gliders are capable of a wide variety of vocalizations.
The "purr" can be found here:
http://www.angelfire.com/nb/sugarglider/sounds/ It's often very faint which is perhaps why you may not have heard it from your little guys. It's somewhat different than a cat's purr, I find. The beats in the glider purr are much slower and more spaced out than the cat's purr. It's rather gentle and sort of sounds like a heart beat. You may notice your glider making this or similar sounds while it's eating it's favourite food, also. <img src="/ubbthreads/images/graemlins/multi.gif" alt="" />
Mine does it every morning to greet me or whenever I move my hand into his sleeping pouch. <img src="/ubbthreads/images/graemlins/agree.gif" alt="" />My Dante's greet is standard - he'll start to purr as he begins by giving me kisses or grooming me (or simply licking my salty-tasting hands... I haven't decided which one it is yet, but I'd like to fancy he was kissing or grooming me) for several minutes; he'll stick a scheduled bite or two here and there to get me to play; then he'll crawl up into a ball in my palm to sleep for the next few hours (sometimes he'll be in mid-play and he'll sleep in whatever position he was in at that moment, arms out and everything!.. lol). That's when the purring stops.
Though my glider now does this, my last female glider, not fully bonded, never purred, but she did make some similar and more interesting sounds. I suppose it depends on the glider. Perhaps to some gliders purring is synonymous to other sounds like "chitting" or quiet "hissing".
Just listen closely the next time you all interact. Listen as they interact with eachother, too. Their language system is quite distinct.
Mikey <img src="/ubbthreads/images/graemlins/dance.gif" alt="" />