FFT showing evenly spaced signals when tuned to ~56 MHz down to 25.680 I'm running GQRX on Mac OS X 10.11.6 hooked up to an RTL-SDR dongle.  I noticed when I'm tuned anywhere between ~56 MHz on down to 25.680 MHz I see evenly spaced signals on the FFT.  But when I tune to 25.679, they disappear completely.

On the higher end, the signals are faint (here I am tuned to 56 MHz):



They become more pronounced as I work my way down (here I am tuned to 43 MHz):



They are most pronounced when I'm tuned to 25.680 MHz.  But when I tune to 25.679 MHz they disappear entirely.  Here I am switching back and forth between 25.679 and 25.680 MHz:



What's causing this?  Since I see signals below 25.3 MHz when I'm tuned to 25.680 but they disappear when 25.679 I tend to think this is a bug.  If so, is it hardware or software related?

EDIT:

The signals are more pronounced tonight.  Each one has a valley surrounded by two peaks:



The one I'm tuned to in the image above has peaks at 27.03 and 27.05 MHz.  The first peak is at about -60 decibels and the second one is at -50.
These periodic spectral lines look like they might be caused by something like a sparking power supply, DC-DC converter (for LED lighting or USB power, etc.), or high powered digital drivers running at approx. 150 kHz. You might want to walk around the house or neighborhood with a pocket AM radio to see if you can DF this RF noise (determine the direction its coming from). 

RTL-SDRs USB dongles based on R820T2 + RTL2832U also have a lower frequency limit somewhere around 52 MHz (where they go deaf or alias or won't lock).  26.68 appears to be near a folding point in the sample rate aliasing, e.g. right around Fs/2, where any spectrum plus its own complex conjugate can partially cancel.