So loud was the rush in her ears?

Galia thought she heard him sniff, loudly, but then she thought this was merely her imagination, too, so loud was the rush in her ears.

Hi. I read this sentence in a novel called "Dishonored". I don't quite understand the underlined part. I think it should be "so loud (being) the rush in her ears", shouldn't it? What construction is it?

Thank you in advanced.