"嘎扎嘎扎的咬嚼" can be translated into English as "the crunching and munching sounds (of biting/chewing)" or more vividly as "the crunchy, munching bites".
If you want to emphasize the onomatopoeic aspect and the continuous action, you could also say:
"The crunch, crunch, crunch of biting/chewing"
"The relentless crunching and munching of teeth" (though this is a bit more poetic/dramatic).
For a more natural and concise phrase in everyday conversation, "the crunchy biting/chewing sounds" would work well.