i have a 2005 chevy 1500 with the 4.8 and this morning when i started it the pressure was good and i went inside the house to let it warm up came outside to the pressure at 0 and the lifters tapping really loud but when i drive it and the rpms are up the oil pressure is right below 40, oil pressure only goes to 0 at idle any help about what would be causing this would be appreciated