When I first bought my X I was leaking a few drops of oil from loose valve covers and anti freeze from a blown head gasket. They tore down the engine and when I got it back my X ticked like a bomb because it didn't have all that oil build up because they cleaned it out during the tear down. Needless to say it took a few days to quit making this noise.
My X:
As you can tell it sounded bad but it was just the lack of oil build up around the lifters. My xterra makes a very faint tapping noise upon start up for a few seconds then stops...I have been told its normal. I would say take it to the shop and let them look at it and not run test so they don't charge you for crap that doesn't matter.
I wouldn't call that normal.
Acceptable? Maybe. Tolerable? Probably. But definitely not normal. Yours is clearly acting like it has a bad valve lifter. When lifters wear out or are mistreated during disassembly, air bubbles can form inside the lifter(s). When the engine is started, the oil pump forces oil up through the block and into the heads. Once the oil reaches the heads, it's channeled to the lifters through the valve guides:
The valve guides deliver oil to the lifters through a small hole in the middle of each lifter (geeze my hands look fat in this pic):
If an air bubble forms inside the lifter, the heads will "tick" until the lifter fills up again or the oil forces the bubble into a place where oil can flow around it. A lot of people live with this defect for years without problem, but it can be fixed.
-Rok