I have a 99 tahoe that is making a ticking tapping noise whenever it sits for a while and I start it up. When I get to riding it stops making that noise. It only makes it when it has been sitting for a while. The oil has been changed and everything seems to be fine other than the noise. So any idea what it could be?

 

Could be a lifter that bleeds down when sitting, this is fairly common on older engines and other then the annoying noise it usually doesn't harm or make the engine any less reliable.
I normally would not recommend that you use a solvent motor flush but in this case it may help, just be sure to follow the directions on the container.