I have an 01 LS and it had started to tick on me maybe a month ago, I changed my oil and everything, and it had stopped for the most part. Now, for the past few days, its started doing it again, only now it seems to contribute to a loss in power and sounds more like someone knocking on a hollow piece of hardwood. I'm going to get an additive, like Ristone, later to see if it helps, but I've honestly run out of ideas, and I'm hoping someone here has some insight into this

 

Check your oil, make sure it has the right amount. But if the knocking is coming with a loss of power it could be more lower end than just lifters. Try and listen and see if you can tell whether its from the top or bottom of the engine.