I have a 4 wheel drive truck with we 200k miles. When i’m driving at a low rate of speed and release the accelerator, I hear a clicking/rattling sound coming from the center part of the truck.
Could be a U joint as a thought, you might get really lucky and just have a heat shield that needs a tack weld.
Does the clicking go away when you stop the truck? If it does, likely u-joint as @Barkydog said. If it still clicks, it may be an actuator from the HVAC system has broken and sits there and softly goes click-click-click.
Exhaust system or driveline problem is where I’d start with that one. Inspect the exhaust system for anything loose, and make sure there’s no play in the driveline u-joints. Good idea to check all the fluid levels, esp the transmission and transfer case.