As Mr.Camn points out, there is a lot going on when you switch from pre to post or vice-versa, so it will not be totally silent. There are fades on the changes so it should not be too noisy.

It would be interesting to know if you get the same thing with nothing plugged into the inputs – it is possible that you are feeding it DC or some steady signal. You certainly can't expect a clean changeover if you are playing at the time of the preset change.