Welcome! You may have a couple of things happening. 1. It could be the upper intake manifold burned through at the EGR port---this is generally accompanied by white smoke out of the exhaust. 2. The lower intake manifold is leaking--check the oil. 3. The coolant elbows to the lower intake manifold like to break and leak. 4. Leaky radiator. Of the things I stated, numbers 1, 2, and 3 are the most likely culprits.