I have a 2003 gmc sierra z71. Recently i have started to hear a ticking noise in the engine. Usually this noise starts after I have been driving for a bit. When I first start my truck everything is normal and after about 5 minutes of idling a very faint ticking noise will start and it will gradually get louder. This noise also occurs while im sitting at red lights. Sometimes when im driving I will hear it but then a few minutes later it will go away again and I wont hear anything at all. I can also go a few days without hearing it and then a few days later it will start again. To be more specific it sounds like its either coming from the 1st or second cylinder. Ive had people tell me that its the lifters and ive also had people tell me its the oil pressure. Its a 5.3L vortec. Any ideas on what this could be?