TAPPING SOUND WHEN STARTING AND USES ALOT OF ANTI FREZE

most likely either a crack in your cylinder head or a leaky heater core, as far as the coolant goes. a head crack could leak coolant into the oil so definitely check and make sure its not milky. as far as your heater core- it would cause the passenger side windshield/window to fog up usually. you might even find some coolant in the passenger floorboard. 2 gallons over 30 days is hardly enough to make a puddle so you could still have a leaky radiator or cracked hose or somethin along those lines. not sure about the tapping, need more info. definitely not sure if they are related.

Well there buddy I've got bad news if you can't find a coolant leak anywere either inside or outside the vehicle its obviously goin into the motor so your head gaskets are popped I doubt the head or block is crack ( GM motors rarely have this problem unlike ford and dodge) you should have a strang smell and heavy white smoke coming from the exhaust if your using that much fluid and the tapping noise is more than likely is a lifter gets soaked with antifreeze when sitting and takes a while to pump up with oil (hence it stops in 10 minutes) so either way you've got antifreeze in the motor and thats a verry verry bad things which is odd for a chev truck either its got a billion miles on it or its been beat on heavliy before you got it DRAIN THE OIL (IT'LL LOOK LIKE A MILKSHAKE) PUT FRESH OIL IN IT BEFORE YOU SPIN A CRANK BEARING AND GET THE HEADS CHECKED BEFORE YOU REALLY BREAK IT

Sorry to say both of your issues are hard to diagnose over the internet. As to the leak, get your system pressure tested at mechanics shop. If it's a small pressure leak you'll likely never find it, but it will show itself readily under pressure... usually. Tapping on start up is not uncommon, especially on larger engines and vehicles with hydraulic lifters... I believe you car fits both... However this is normally not LOUD as you put it, so you should probably take it to a nearby mechanic to see what they think... it's nearly impossible for us to diagnose a sound over the internet after all. Long story short... Take it somewhere to be diagnosed properly.

If it just started doing this, do not drive it. I know this sounds stupid, but I need to get it out there. Since the sound goes away, I would rule out a blown head gasket or a cracked head. GM on the other hand did have a HUGE issue with faulty intake gaskets in the newly designed V8 and V6 engines until about 2006. They had a tendency to leak coolant down the back of the engine block. Due to the location it is very hard to see, and the amount of coolant that is being lost is usually always evaporated. I would check with your local dealer as there was a recall on a select few models for this problem, although it was a larger problem then they wanted to acknowledge. Your engine tapping at start up is most likely an entirely different issue that is either being caused by either high mileage taking its toll on your oil pump OR the weight of oil used in the last oil change being too heavy.

There is no coolant that runs through the intake of the V8s - the intakes are plastic (since 2000) and only coolant runs through the throttle body mounted to the front of the intake. so it's has nothing to do with the inatke gaskets. There has never been a recall on the intake gaskets of the GM v8's in trucks since the late 90's. what did sometimes happen, is the coolant transfer tubes that run UNDER the intake would tend to leak where they were mounted in the corners. this was later redesigned and the tubes are no longer used. The guy has a blown head gasket or a cracked cylinder -

Most likely a head gasket is bad. You may be able to pull all the spark plugs and look at them closely and determine which cylinder it is. If you find one plug that is much cleaner than the others then that cylinder is the problem and the head gasket on that side needs to be replaced. Note this only works sometimes and only works when a massive amount of coolant is going through a cylinder. For proper diagnosis pressure up the coolant system with a tester and determine where the coolant is going. If is not dripping onto the passenger floor, not dripping underneath the vehicle,and not going into the oil pan it is going out the tail pipe.

My 1997 GMC Sierra pickup truck started using up antifreeze a couple of years back. There was this sweet smell coming from the exhaust system while the truck was running. I didn't address the problem and one day while traveling down the highway to work my truck suddenly over-heated badly. My mechanic had to replace my head gaskets. Problem solved.