Started the truck this morning (2004 GMC Yukon 5.3L Flex 206k miles) and immediately noticed a knocking sound (with doors closed and windows rolled up). That was unusual so I shut it off, got out of the truck, opened the hood, and looked for anything obvious. I noticed the coolant reservoir was low but it has been lately. I've been planning on changing the intake manifold gasket because I couldn't find any other coolant leaks... Did my time run out? I checked the oil and saw that it had a few bubbles in the oil towards the top of the stick. When I went to wipe the dipstick it was noticeably sticky, not how oil should feel. I've never had this issue before but I know it's bad news. It happened to my dad's 98 Suburban. So what are my options?