Crowded classroom / playground = noisy. Quiet classroom / backyard = quiet. That's a pretty basic connection there. Furthermore I will add that she may have a speech sensory issue, not just straightforward "noise", or only certain kinds of noise. My DH cannot cope with noise that I am able to filter out an keep functioning (even if I don't like it), I have other kinds of noise I find harder to filter and get tired/cranky very fast. But speech is my major auditory trigger. I cannot cope with multiple people speaking at once, especially multiple people I "should" be processing simulataneously, that i can't just ignore. Makes it very hard for me to be in the same room as all I my children and husband at the same time :-). If she's trying to process every conversation in the room/playground she may be overloading....
As for food allergy and intolerance present differently, intolerance is a load issue and there are good chemicals that are present in foods you would never think of as connected (oranges, chocolate and cheese are all strong amines). So I would not rule out food either.
