WAter Sloshing in dash

like it says when i drive i have this water sloshing sound in my dash, any ideas?

most likely the heater core has some air in it allowing water to gurgle in it as the water pump circulates coolant.
if its a dexcool car (red antifreeze) refill ASAP. dexcool does not like air in the system. causes sludge...
dexcool works great when its a sealed system.
what year car?

Most likely the heat core is blocked.
 
Do you have any coolant leaking on the carpet?  Furthermore, how is the heat blowing?

Try again guys.  The heater core is not to blame.  No real issues with those.  The water is most likely from the outside air filter.  There is a seal along the bottom of the windshield that comes loose and allows water into the dash area.  Has your apssenger floor been wet?  Replace that seal.  Any Chevrolet Dealership service department should know which seal you are referring to.

I have wet carpet also, passenger side, 2000 LS with dual heat controls, only 60K miles on it.  It is certainly coolant, smells funny, looks funny, orangish and slimy.   Just last week the dealer replaced the ignition switch, which is very close to the heater core, certainly makes me wonder if they damaged something.