According to the in-game help on Sneaking:
Sound: Attacking or casting spells makes noise, which may alert enemies to your presence. Heavier weapons make more noise. Spells produce both sound and light.
By "heavier weapons", does this mean actual encumbrance weight (so daedric daggers make more noise than iron daggers), or only the size category (so two-handed weapons make more noise than one-handed, but all daggers are equally quiet)?