Hi Paul, I'm Clay a certified master automobile technician with 30 years of automotive repair and diagnostic experience.
Let me start out with replacing the lifters is not what you would call a walk in the park. The intake and heads will have to come off to get to the lifters. Book time calls for about 11 hours. I have all the service procedures if you need them. I can offer an addition service for 5 bucks that will allow us to communicate through regular email. It's much easier to send this much information through regular email than through the site.
You may want to manually verify the engines oil pressure. The GM service procedure for checking the engines oil pressure with an external oil pressure gauge is in the link below.
Oil Pressure Diagnosis and Testing
Oil Pressure (Minimum - Hot)
At 1,000 RPM 6 psig
At 2,000 RPM 18 psig
At 4,000 RPM 24 psig
I also have a service bulletin concerning lifter noise or an engine tick noise caused from an 0-ring seal between the oil pump screen and the oil pump the seal may be cut and cause aeration of the engines oil. The TSB is for low mileage vehicles, but if the seal is leaking it really doesn't matter how many miles are on the vehicle the noise would still appear. I'm just not sure the problem was addressed when the vehicle was new.
Let me know if you're interested in the TSB and my additional offer if you're going to replace the lifters.
I hope this information is helpful. If you have any questions or run into any problems let me know.
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