Here's an image of it:
It kind of looks like ダダ (da da) to me, but then I'm not sure what to make of the last mark. Could someone offer his insight, please?
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It's either ダダー or ダタッ. Given that if it were a ー on the end then it would probably be horizontally aligned with the other characters, I'm going to assume it's a ツ, and because ダタツ doesn't seem to make much sense as onomatopoeia, it should be a ッ (small-tsu). Given that it looks like a kind of shorter 'impact' sound and not something extended or ringing, I think we can rule out ダダー on both counts.
So my vote is for ダタッ!