Is it different than her "hysterical" cry? is she a "crier"? Some babies are not as loud as others.
It also depends on the age. When ds was about 4/5 months he'd do this "high whine" (a lot like you're describing) at the 50 min mark of his nap. It was on and off. I tried letting him go for 10 minutes and it turned out to go to sleep again. He did it for a few weeks (for each nap) and then suddenly stopped. IT was like Mantra Cry, some noise he used to self soothe (it did NOT seem soothing to me AT ALL :roll: ) but I could tell he didn't need my help at all (even though the times I came in, he received me happily).
I'd say just watch her, establish a time mark (say 10, 15 min) and observe him from outside her room to see what she does: does she seem upset? umconfortble? are her eyes opened or closed? how long has she been sleeping before this happens?
Tracy claims not to leave a child cry out alone but for fussy cries, I think it's OK as long as the crying does not escalate.
HTH!