I have heard that noise on a lot of 2.5 3.0 modular ford v6.
What I sometimes find is the end cylinder 1,3,4,6 has a broken exhaust manifold bolt. They are sometimes in the hole but broke off not holding. And since the gasket is metal and multi layered it makes a puff\ tingny sound. Stops when warns up because the metal swells.
I would check it