Sounds like the exhaust manifold to me. Mine was making a similar noise. The manifold warping is a common problem with these. You'll have to pull it off and get it machined. It's a bit of a root around. Not expensive to get it machined. Look around online for a gasket kit.
Being that it only does it when it's cold makes sense because as the heat expands the manifold the gap will close up and re seal.
