Well, it looks like I have joined the head gasket club. Had an unusual whistle coming from drivers side and it was the overflow tank. Lost a little coolant, didn't overheat. Topped off tank after letting truck cool for a few hours in driveway, and presto! Instant white smoke on start-up and you can smell the A/F burning. Dropped off at shop, keeping fingers and everything else crossed. No missing, ran fine...can't wait to see this bill.