2004 Mercedes Benz C230 Air Conditioning Hissing Noise and

Hi,

My 04 C230 (35,600 miles) has the ability to have the drivers side a different temp then the passenger side.

I currently have a hissing noise under the dashboard and the drivers side vents are blowing warm/cool air while the passenger vents are blowing cold air.

Any ideas on what this can be?

Thanks
~Rx.