I would definitely narrow down where the odor is coming from. If the 'sour' odor is coming from her mouth, I'd strongly advise you to seek out an avian vet as it could be signs of a crop infection.
If its her feathers, then it could be one of two things. It could be just her natural scent OR it could be an 'absorbed' scent from her environment.
For example, if Skittles takes his 'morning dump' inside his sleep cage, there is a STRONG odor and if he spends a significant time near it, his feathers will absorb the odor temporarily. He has done that a few times (done a morning dump in his sleep cage) but its when I've overslept. Normally, his morning dump is taken on one of his playstands (usually the one by the window).
As for natural scent, when I had my cockatiel, Peaches, she smelled like maple syrup and when she was wet- the smell was EXTREMELY potent and you could smell the 'maple' from several feet away.
Skittles breathe smells like 'cake mix' and its mostly due to his diet. When he regurgitates his food, it has a strong odor of raw cake mix and the scent just permeates through his feathers. As a result, he smells like cupcakes.