Some cars will make a swoosh noise if the brake pedal goes down lower than normal. I think it is the rubber boot on the booster sucking or blowing air when it gets overextended. I used to notice it sometimes when I was the brake "pumper upper" during a brake bleed job.
First off,you might want to check your rear brake adjustment. The HHR is notorious for the rears to get out of adjustment. This might get your pedal height back to normal. If not,then on to checking the master cylinder or booster.
Let us know what you find out.