New video part causes crackles in sound

[noiser]

I have some problem with Openshot - in a lot of short movies which I made I have some subtle but audible crackles in sound. Here you have an example: https://youtu.be/WSroc7Jgb74?t=44 -> starting from 0:44 it is audible a small crackling noise when the new part of video is set in path. Do you know this issue? Someone had it already before?
I was thinking that it depends on my drivers but I re-installed it again and problem exists.
Thanks in advance.

Details:
Architecture: 64 bits
Operating System & version: Ubuntu 16.04 LTS (upgraded from 14.04 LTS)
Information about Openshot, MLT and FFmpeg versions I can deliver later (far from the machine at the moment).