Panic attacks "out of the blue" send body signals up to an hour earlier.

Science Daily News:    A high-risk group for panic attacks was monitored day and night.  Thirteen attacks were recorded during that time.  The surprise discovery was that about a dozen body and breathing changes take place during the hour before. The subjects were unaware of this.  Getting the message earlier may prevent attacks.
This article is short on specifics about the physical changes.