Jimmy was trying to ignore the rattling sound she was hearing that seemed to be getting closer while he stood by to watch this all unfold. He shook his head once and as soon as he saw Castiel open his mouth - he knew he was going to try and say something because it was the same face Jacob made when he was about to.
So he shushed him. Their Castiel had this under control, he thought. There was no need to piss off the archangel anymore than he already sounded.