Adverb (fukushi), Adverb taking the 'to' particle, Suru verb
gasping for breath; puffing and panting; wheezing
Onomatopoeic or mimetic word
ぜいぜい言いながらも、気合をいれて走り続ける。Puffing and panting we continued to run with renewed vigor.
Also written ゼイゼイ、ゼエゼエ、ぜーぜー、ぜいぜい、ぜえぜえ、ぜぇぜぇ