I have an 05 Tahoe Z71 with 17,500 miles on it that is bone stock. This thing spark knocks when I go 3/4 throttle or more unless I have 93 octane in it. Is this a common issue or is there something I need to worry about?

Another issue that I'm having is, when I first start the Tahoe up there is a "thumping" noise that comes from underneath the truck (sounds like its coming from the transfer case area). It sounds like something is in a bind and it is freeing up. The thing that makes this odd is that I haven't even put the vehicle in gear yet and it's making this noise. Has anyone heard of this problem?