What part of speech it is depends on the context. If the situation is someone saying something like, "Hallooh! Is anyone out there?", then "hallooh" would be an interjection.
Otherwise, probably a noun … “As Rose floated in the debris field, she heard a faint ‘hallooh’ and then saw the lantern on the lifeboat.”
This makes me think of “Mary Poppins”, haha. But yeah, Navigator is right. If someone is shouting “Hallooh”, then it’s an interjection.