My 2007 escalade with 60,000 miles has low oil pressure (8 psi) after driving down the interstate (low idle). Knocks and sounds like a clunker only after driving down the interstate. It sounds fine driving through the city. My mechanic replaced the oil pump and sensor a couple weeks ago. It does not show the warning light any more stating "low oil pressure, stop engine." anyone have any ideas? Needs new lifters?