My 03 tahoe is clicking when I start it

You need to have your heads checked out. There are a couple of ways to do it, the first being, have your oil changed, and see if there's evidence of coolant in it. The second, you can do yourself if you wish, but you need to pull your valve covers and check for a thick, sticky yellow substance on the underside of your valve covers. If you see this, you have most likely a cracked head on your hands. It is fairly common with these, as there was a bad set of heads made for these engines. What happens, is this sticky goop is from water getting into the oil. It plugs a lot of things up, including the oil pressure sensor, and your lifters, resulting in oil pressure fluxuations (or very low oil pressure as seen on the gauge) and lifter clatter when you first start it up. If your tahoe has low mileage, you may be able to get a dealer to split the cost with you, as it's a common problem. I hope that I'm not right for your sake, but I'm in the process of changing the heads on my '01 suburban with the same engine as you have in yours for the same reason.