You're right that the word comes from Old Norse. The words that come from the North are always the most colorful in English. You understand its meaning but it doesn't work in this sentence for grammatical reasons. "Berserk" is an adjective, not an adverb. An adjective cannot modify a verb. In this sentence, it is being used to modify the verb "rattle", so that is an error. It is OK, though, as an example of colloquial English.