small distortions in the frequency spectrum while editing

Are such small distortions in the frequency spectrum normal?
They appear when trimming, silencing, low pass filtering audio…
3tracks-distortion.png

Yes they are both normal and undesirable. When you play that region you will hear a click where the vertical line is.
If we look at a close-up of the waveform where one of those vertical lines is you can probably work out what has gone wrong.

Here I’ve generated a 700 Hz tone (it is a lot easier to see the problem with a lower frequency) and selected a section from the middle and deleted it. The result produces a vertical line almost identical to the ones shown in your examples. Below is a close-up of the waveform view.
firsttrack000.png
As you can see, the region that the cut section began was when the waveform was at a high point, and the end of the cut region was where the waveform was at a low point, so after deleting there is a sudden jump which represents an instantaneous burst of noise, produces a red vertical line in the spectrum view and can be heard as a click.

In this particular case the problem can be easily avoided by pressing the Z key before deleting the section. This will snap the selection to the nearest “zero crossing points” (if they exist, which will of course they will with mono sine tone). By selecting between zero crossing points, the two remaining halves after the deletion will match up exactly and there will be no red line and no click.

If you look at the waveform view of your 7kHz sine wave after filtering, you will notice that the amplitude suddenly drops. As in the previous example it is this sudden discontinuity in the waveform that produces the red line and the click.

The discontinuities that are caused by filtering are a little more difficult to avoid. Often the easiest method is to choose the edit points carefully, for example to start and end the selection at moments where the track is near to silence, or immediately before a loud beat in music. The first option minimises the problem, the second option hides it. If neither of these options are available then you need to copy the region that is to be processed, then do the processing on the copy, and then cross-fading from the original to the copy and back again.

Thank you stevethefiddle. Your explanation is superb.
I hoped that someone will shortly explain me the source of these distortions. You did that in much detail and hinted me at a solution, which works well enough for my needs (‘Z’).

Now that I know what to search for, I see that the manual features a description… Perhaps the glossary could mention “zero crossings” as well?

http://manual.audacityteam.org/index.php?title=Glossary#zero_crossing