Hartmut,
I don't think that you have a ground loop; I think the LCD on your laptop is the culprit.
Try turning the LCD off-- all the laptops I've owned will turn off the screen if you close the lid, but you may wish to double check the power settings in the control panel-- some laptops will suspend when the lid is closed, and that wouldn't help.
Basically, start the recorder, and then turn off the LCD. I suspect that the noise will go away, even if plugged in.
What I think causes it is the ballasts for the cold cathode tubes that backlight the LCD.
I don't think that there is much you can do other than isolation and shielding:
Isolation:
Isolate the AC mains by physically separating them. In the US (residential), we have two 120V buses in the power system, 180 degrees out of phase, and putting them on different buses might help if you can figure out which bus is connected where in your house. That may not help, since they share the same grounded connector. The grounding (neutral) connectors are on different buses, but are tied to the grounding connector at the ground point (all buildings in the US have a single ground point).
Of course, you have 230V @ 50Hz, and I'm not sure if you have separate buses in your panel or not. Oddly enough, Germany hasn't adopted the US electrical code (I'm kidding!).
Shielding:
You could try to cook up a faraday cage (look up faraday cage or tempest shielding on Google-- these are both related to Van Eck Phreaking, which I referenced earlier). You may need to figure out a way to ground the cage; this is something to talk to an electrician about, since you can be injured or killed by hooking things up to your electrical system.
In short, turn off the LCD while recording. I can almost guarantee that will fix, or at least markedly improve, your issue. Especially since you can hear windows moving and whatnot.
Brad
