No, not necessarily "constipation"...sometimes you just hear normal gastric "sloshing" when you handle a snake (esp. when too soon after a meal for handling).
I agree with what the store told you...keep monitoring. "Whistling" could be an R.I. or could just be some water or old shed skin that's partly blocking the nostrils.
Time to "play detective"...& it's probably still too soon to panic.