Well, it is either Norma or Harold. Wikipedia says it is Norma, but now that you mention it, I also do remember Donna saying that. Not that it matters much.
Regarding Norma's motives? I do not really think Norma need any motives. She is properly the least scheming character in the entire show. She properly just overheard the group discussing what to do about their investigations, and just decided to give them a hint.