What causes a tapping noise when starting the engine?

The tapping noise while warming up in the Grand Am GT's is nothing to worry about. These vehicles have the 3.4 Liter V6 engine and due to the design of these engines, it is simply oil entering the engine that causes this sound. If you noticed once it is warm the tapping ceases. Like I said need not to worry because this is common.
I would not be driving this vehicle for a couple of reasons. If you are loosing coolant on a continual basis, regardless of how much is lost, it has to go somewhere. If you do not see any pooling under the vehicle, then it is leaking inside the engine. If the latter is true, then coolant in the engine will dilute the oil, and create a cloudy oil residue, or if you are getting oil into the coolant, you will see a 'rainbow' on top of the coolant in the radiator or overflow. The tapping, I think, from the limited information, is the Valves not getting the Oil due to low oil pressure. Once the engine warms up, and the Oil pump starts to flow the oil, the tapping stops. Many vehicles, especially GM and Chevy do this. I would not be overly concerned about the tapping, but I would be concerned about the coolant leak. An easy way to see if you have an external leak is to go to a local parts store, and purchase a leak test kit, which usually is a dye, which can be seen with a blacklight. Kids love helping at night with this! Good Luck it might be your water punp you need a stronger one. or nr oil too the head Adding to answer: Take vehicle to repair facility and have a Carbon Block Test done,it will also determine if Carbon Gases are entering the coolant system.As far as the tapping, switch to synthetic oil,it costs about twice as much but is well worth the expense. http://www.autozone.com/servlet/UiBroker?UseCase=S001&UserAction=viewSimpleDiagInfo&Parameters=info answer does the noise go away shortly after starting? if so the tapping noise is caused by cold and insuufient oil in the lifters on your tappets as the engine stands, not running, the oil that was circulating while running, goes to oil pan on starting, the oil needs to be circulated back to all parts, try an oli change with correct viscosity oil sometimes a good oil additive such as LUCAS ENGINE OIL ADDITIVE will stop this rattle all cars will have same problem as they get older