Tinny sound when I start up my truck

Could just be a loose heat shield from the exhaust header or on/near the cat.

Possible exhaust leak.

Yeah that's a better suggestion, especially if the noise goes away as the truck warms up.

Could be a bad exhaust gasket, cracked exhaust manifold or a missing exhaust manifold bolt. As the engine and manifold warms up, the material in the area of the leak expands and the leak stops. Could also be a loose spark plug. Has any work recently been done to this vehicle? Which engine do you have? Miles?