2005 Chevy Impala Sloshing sound behind dashboard?

Hi all~ after googling "sloshing sounds dashboard chevy impala" and coming up with a consumer complaint website/class action lawsuit against "DexCool" engine coolant- I decided to call the dealership where I purchased my car and ask if they used DexCool coolant. The answer was a resounding "yes!". My car is making a sloshing sound behind the dash in the mornings when it is still cool outside, and I have no indicator lights that my fluid is low, nor is my thermostat going crazy saying it's overheated....I checked the fluid color and it's starting to turn brownish-so the dealership is probably right about the fluid level being low and needing to be changed/filled. I just want your take on it-because I can't seem to find any information on my car anywhere else. Thanks.