I will disagree on that statement. I have heard the conductors SLAP in the conduit when energizing an unloaded transformer. I believe it is called magnetizing current.
In essence, the inductive reactance of the transformer is zero until the current starts flowing, therefore the transformer looks like a short circuit upon energization. A large current starts to flow, which produces strong magnetic fields, which pushes the conductors away from each other, resulting in the slapping of the conductors against the conduit.