I have a 2003 z71 1500 5.3 vortec suburban that recently started dropping oil psi and tapping in the valves. Sadly I waited 9,000 miles to change the oil and filter last week and know when I start it up it reaches 40 psi max and after 5 minutes or so it drops to 20 psi, low oil pressure alerts then the valves tap away. Is my oil pump shot??