I just bought a 2008 gmc 1500 4x4 with 5.3 litre and was driving it home (440kms), not knowing the truck had AFM, I heard a sound like a solenoid turning on, then a noise from engine like clicking or knocking. Then noticed low oil pressure. I pulled over and turned it off and checked oil level and it was ok. started back up and pressure was normal and sound was gone till I got up to 100km/h and sound came back. I shifted from drive to 3rd and sound disappeared. is this the lifters or what. is this cause by the AFM?