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Is it a Gen3 or Gen4 engine? If it is a Gen4 with AFM/DOD (Cylinder Deactivation) my guess would be one or more of the 8 AFM/DOD Collapsible Lifters is starting to fail. If so best thing would be is to remove all the physical hardware for the AFM/DOD system. Which would be the Cam, Lifters, Lifter Guides, Pushrods, Valley Cover and tuning it to remove this feature. Most failures about the AFM/DOD lifters I hear about fail around 130-150k miles. They start with a faint tick and continue to get worse until the fail, which usually takes out the cam and could cause other issues. You can purchase a AFM/DOD removal kit from Texas Speed for relatively cheap as cam.

Now if it is a Gen3 in a Silverado Classic, it may also be a lifter failing which you would need to just replace the lifters.
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