Now, I am only going on what I am told here, not what I know as I am not an electrical engineer…
I discussed this in the past with a couple of electrical engineers we have on our team and their answer was that AC to AC transformers don’t introduce noise, or at least, not to the point where it matters. AC to DC transformers are the ones that create noise.
They went into a lot of detail but basically they said that the power you receive already goes through many transformers that reduce the voltage on its way from the electrical station to you home/work/whatever. Tens of thousands of of volts are reduced along the way until it is the final 220V (or 110V) and because the it is still AC, the wave forms stay the same.
They say that having a simple AC to DC transformers plugged in somewhere on your circuit at home will introduce more noise than any of the AC to AC transformers, unless the AC to AC transformer has an issue.
Again, this is only what I am told so do not take this as fact without research.