Anyone with this problem needs to change out their thermostat. Super easy there’s a couple YouTube videos on it. Iv done it twice as the first autozone T stat failed.
Iv been running the CST 4.5 lift with another 2” Bilsteins in the front, 2” block in the rear with 35” BFG KO2 for the past 3.5 years no issues couldn’t be happier. I would say ride quality is similar to stock but you can see two to three cars ahead over the top of cars and trucks ?
So I too have that noise however I don’t believe it’s the flapper valve because I don’t have one with the Borla cat back. I notice it got louder so I changed back to the stock tune and it seemed to go away, but eventually came back. I’ve only noticed the noise when the truck is under a load. The videos in the previous post sounds like noisy lifters which is a known issue with Chevy/ GM