Could it be the coolant reservoir
If you have the Northstar V8 it's going to be the head gasket. Have the cooling system checked for combustion gasses to confirm this. HTH. -Jim
Reservoir tank was my problem on my 2000 Cadillac DeVille it is not always the head gaskets but they are notorious for head gasket problems also make sure the purge line is not stopped up coming from the reservoir tank back to the water pump housing
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