2002 Chevy Silverado Coolant leak and water sound behind pa

I am using about 1 1/2 quarts of coolant every two weeks. I can find no visable sign of a leak in or out
of the vehicle. When I start the vehicle, I hear within
the cab, the sound of water, and possibly air, in a gushing sound which eventually goes away as the
vehicle comes to full operating temp. I do not smell
any odors within the cab or under the hood. I am not using/loosing oil. The temp guage and lights do
not indicate any overheat. There are no sounds out
of the ordinary, no squels, nor metallic knocks.
Nevertheless that coolant is going somewhere. This
started about 60 days ago. My driving is short haul
city, about 75 to 85 miles weekly. 20% would be on
60 mph freeways. Any ideas would be helpfull.

Thank you,
SkyTeam65