Hi, there is a tsb(technical service bulletin) that talks about your concern. This may be the problem.
#05-01-38-016B: Water Leaking on Front Passenger Side Floor of Vehicle, Water Blowing Out of Vents (Seal Heater Core Cover) - (Aug 16, 2006)
This bulletin is being revised to provide the part number of the screw that is required for the Cobalt and Pursuit models. Please discard Corporate Bulletin Number 05-01-38-016A (Section 01 -- HVAC).
Some customers may comment on water leaking on the front passenger side floor of the vehicle. This leak would occur under high ambient temperatures and high humidity situations when the customer has the air conditioning on high. Leaks will appear to be coming from the HVAC module where the heater core cover and lower case come together. Also, some customers may comment on water droplets blowing out of the vents when the A/C is on high.
This condition may be due to water condensing in the HVAC case that adheres to the upper surface in the tongue and groove area of the lower case and then leaking out.
Install the foam gasket between the heater core cover and the lower HVAC case at the location of the leak using the procedure listed below.