Did you paint the manifold? Even hi-temp paint will burn off to an extent. It will smoke. I'd also check for a coolant or oil leak. If it is something that wrong you will see a puddle in the driveway. Also, don't assume you kept everything clean since just touching it can get enough residue there to make it burn off on a hot exhaust manifold. If you find nothing wrong let it run for 30 minutes.