2008 Chevy tahoe 5.3l engine squealing. Engine has 244,000 miles, I replaced the lifters 3 years ago, using a Boost AFM disabler. Have a launch scanner, checked for codes, nothing. Checked belts and accessories on the front of the engine, nothing there. Purchased microphone set and sound is loudest at the rear of the engine around the #8 cyclinder. The sound is extremly loud, my wife can hear me coming down the street. Removed the strater and checked the fly wheel teeth and starter both look good. Removed the passenger side valve cover and checked the rockers, all tight. Ran engine with valve cover off, all lifters are operational. Removed valley cover and checked lifter contact with the cam shaft. Lifters looked good (the ones I can see 1,2,7,8), cam lobes are smooth. I'm in the process of pulling the engine becuase I am out of ideas, any help would be appriciated.