When the 2018 F150 XLT makes a rattling noise when you release the accelerator while it's cold, but the sound disappears when the vehicle warms up, it could be due to several reasons. Since you've already replaced the timing components and the turbo, you might want to check other areas such as the exhaust system, heat shields, or engine mounts for any loose parts or issues. It's also advisable to consult a mechanic to diagnose the problem accurately.