2003 Pontiac Grand Am coolant problem

I have a coolent leak I can't find out where it's coming from. I've replaced my thermostat.
And water pump The coolant leak is not visible I checked oil can't see any coolant don't see anything on the ground heater is also blowing cold air on and off so this morning I filled up the coolant and car drove best kind park it and half hour go to get in it and coolant light be on I poped the hood and seen coolant over flowing out of the hose on top of the coolant case I brought to A mechanic and did a pressure test he said it was losing 2 1/2pounds per half hour he said it didn't look like the head gasket but didn't know what was the problem