There is a rattle up front just after cold startup in the morning. It sounds like the timing chain. I replaced the tensioner with no change. It's worse in colder weather and if I park in the driveway with nose down. Changing from Rotella T 5W40 to Mobil-1 10W30 helped slightly. Motor was rebuilt 15k miles ago. All timing components were replaced but the tensioner. I was going to pull the valve cover and try to inspect the chain guide & slipper, but I figure it would rattle all the time if they were broken. What could it be?