Coolant leak problem

My son has a 1996 Camaro with the 3.8L V-6. It is loosing coolant and sometimes will overheat. I pressure checked it by pumping the pressure up to about 16psi. when the engine was both cold and at normal operating temperature. The system would loose about 1lb of pressure over a 30-40 minute period with no evidence of a leak anywhere. I am puzzled and don’t know what to do. Thanks so much for your help/suggestions. Randy

Any white smoke/steam out the tailpipe? That would point to head gasket.

I’ve seen a water pump that leaked intermittently only when the engine is revving. How old & how many miles is the water pump.

If it’s losing coolant but there’s no visible leak it’s possible one or both head gaskets is leaking and the coolant is going out through the exhaust pipe as steam.

This would also explain the overheating.

A mechanic can test for combustion gasses in the coolant. Their presence would confirm a head gasket problem.

No white smoke and I did have him rev the motor with no visible sign of a leak.

I believe the water pump is original and the car has 107,000 miles on it.