"Zawazawa" (ザワザワ）is the Japanese onomatopoeia for the sound a noisy crowd of people makes. If the given frame in the manga has a picture where such background noise is appropriate, this would be the best translation.

A different interpretation I just found uses zawazawa to describe the crawling of a caterpillar.

Of course it all depends on the context, but since it's not "zawa zawa" but just "zawa", this could be an emotion which is associated with having a bad premonition.