We've also got a light rap when the engine is cold on our '02 Avalanche. I wouldn't say it's normal. To me it's the sound of a worn main bearing. Unless it gets louder, or persists after engine warm up, I wouldn't worry about it too much, but know that in time it may develop into a persistent knock. By "in time" I mean with proper maintenance, you probably won't have any real problems directly linked the to tap for at least another 5 years minimum. But I'm not promising anything either. Be good to your engine, and take care of it, and the tap should only be a minor nuisance.