In GQRX (an GNURadio based RTL-SDR receiver) I have an FFT and waterfall display. When I tune to a local FM station (107.5MHz) I can hear strong, clear voice.
But on FFT and waterfall display I can see also a weaker signal at 107.9MHz:
Why I see this little signal of my receiver is tuned to other frequency? There is almost always such a weak signal in the center of a display no matter at which frequency I'm tuned.