My engine squeals. It only does it when the engine is hot. It's annoyingly loud, louder from underneath the truck. It sounds like it's coming from the back of the block. Sometimes when I turn the engine off the noise will persist for a second or two, like pressure is being bled off. When I'm driving it squeals at lower RPM's and lower loads. If I hit the gas or let off, it will go away. I know it's not the transmission(700-R4) because that was completely gone through this summer. It will do it in or out of gear. The torque converter is new. It started with about 2000 miles on some Castrol synthetic blend oil(15w-40). I have never used synthetic before. I disassembled and visually inspected the oil pump. It looked fine. I put some conventional oil(15w-40) in and it disappeared. Well, now, 2,000 miles later, it started again. So, it's oil related. My mechanic has 30 years of experience and he couldn't diagnose it. Any ideas?