Very quiet when operating in 25KV mode.
More seriously though, in diesel mode the power pack vehicle is louder than, for example, an
IET▸ or Turbo engine, but not by much. And of course the rest of the train is extremely quiet unlike
DMUs▸ with underfloor engines. You might need a larger power pack vehicle, and therefore a little more noise, if you were to provide five passenger vehicles as the current examples only go up to four passenger vehicles.
I don't think power is an issue. The class 755/4 has nearly 2 MW of grunt, and is overpowered by British standards using only three of its four engines. (I think they fit four more for mass balance than anything else.)
What I'm less sure about is whether Stadler's kit of bits of train can do a 5-car bi-mode. But I am sure they can find a way, even if they don't have one already.