Why does my car make a hissing sound when I go over speed bumps?

I just heard it today. My boyfriend says it happens when the shocks or struts are working properly, but like i said, i have never heard it until today.

This is a pretty new car. I bought it in October of '08. Its a 2005 Pontiac G6. When i bought it, the salesman told me it had just had a brake job, but I just found out I need to replace the front rotors. What else could go wrong? Maybe i shouldn't ask myself that, knock on wood. I am planning on taking a thousand mile trip here in a couple weeks. Might have to get a rental... can't afford a rental.

Do you know, is this a crucial thing i should be worried about? Can i make the trip and then fix the issues when i come back? Help!