1999 Pontiac Trans Sport 3.4 litre engine with 15-thousand km. Low water warning light came on on dash several times then stopped eventually when I added coolant to reservoir. Checked level in radiator which is fine. Not overheating and runs smoothly. Once also started to smoke or steam ??? under hood but went away quickly and hasn't happened since. Someone suggested a bad manifold gasket. Any ideas?

Hal