Moan x groan: Qual a diferença

Hey everybody. I've been searching about the difference between these words and I can't find anything about them. Can you tell me? What's the difference between moan and groan?
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Groan, moan refer to sounds indicating deep suffering. A groan is a brief, strong, deep-throated sound emitted involuntarily under pressure of pain or suffering: The wounded man groaned when they lifted him.A moan is a prolonged, more or less continuous, low, inarticulate sound indicative of suffering, either physical or mental: She was moaning after the operation.

Source: http://dictionary.reference.com/browse/groan