Hi Clemens et al.

Thanks for all those hints! Finally I think I've found it.
It's somewhat trivial, should better write embarrassing (sic!).

I used an old 9 Volts-block for testing. Since it ran out of
voltage (<7 Volts) during the trials I changed over to a bundle
of 8 AA-Cells. The effect now was nearly "invisible".

Using a new 9 Volts-block it reappears but not such strong.
Strange.

Adding some capacitors right over that battery of AA-cells
I have to knock very strongly now until the effect is discernable.
Touching the battery does not show any further interferences.

Made some screenshots, distance of both probes approx.
1 m (cave, big!).

The old e-field probe running a BF981:
http://www.df3lp.de/misc/rx/Left_BF981.png
Images: http://df3lp.de/misc/rx/e-field_probe.png

The new INA111 balanced probe:
http://www.df3lp.de/misc/rx/Right_INA111.png

Greetings from Kiel es tu,
Peter, df3lp