Adverb (fukushi), Adverb taking the 'to' particle, Suru verb
trembling (with fear, anger, etc.); shivering (with cold); shaking; quivering
Onomatopoeic or mimetic word
しゃべり出した時彼の両手はぶるぶる震えた。His hands quivered when he began to speak.
Also written ブルブル