Rust from your cooling system is flowing back into the overflow tank when the motor warms up.
Rocket prapelant
It has a compressor (like a refrig or air conditioner) which compresses a gas (freon) creating hot gas, This gas heats the hot water tank as it condenses to a liquid, The liquid then goes to the evaporator, where it boils back to a gas again, taking heat energy out of the cold water tank and cooling the water before returning to the compressor to start again. The hot water tank may have another heat source to boost the temperature higher.
What size gas fuel tank does a 2001 Buick lasabre have
The gas tank on the 2014 Buick Regal holds 18.0 gal..
The gas tank on the 2013 Buick Verano holds 15.6 gal..
The gas tank on the 2010 Buick LaCrosse holds 18.4 gal..
The gas tank on the 2013 Buick Regal holds 15.8 gal..
The gas tank on the 2004 Buick Regal holds 17 gal..
The gas tank on the 2009 Buick LaCrosse holds 17.5 gal..
The gas tank on the 2011 Buick Enclave holds 22.0 gal..
The gas tank on the 2003 Buick Regal holds 17 gal..