Stewart and Matti,

 thanks again for your answers.
Thanks also to Alan Melia who, among other info,
suggested me to browse the Web site of Renato, http://www.vlf.it
who has a list of the stations on that part of the spectrum.

I found the origin of those straight lines. The culprit is
the UPS of my PC. Apparently, to guarantee the exactness of the
generated 50 Hz in absence of the mains, it has a quartz
oscillator, from which also the switching frequency is derived.

Upon suggestion of Claudio IK2PII, I listened to the "cloudy" signal
using a W&G Selektiver Pegelmesser, and, among
pseudo-white noise, it is possible to hear a faint RTTY-sounding
signal.

The 15625 peak, as suggested by Stewart, is there within just a
small fraction of an Hertz. When the TV upstairs in my house is
on, the peak is very pronounced. If I switch off my TV, I still see
a reduced peak, which with most probability is coming from the
house of the neighbours (50 meters from mine). I can even watch
when they change channel, from the slight discontinuity in that
signal...

73  Alberto  I2PHD