Edit: i found a better solution: TI2 Sine Without pop
This phenomenon occurs in most synths and samplers. You can try other synthesizers (hardware or software - it doesn't matter). Even if you record it, for example, in Ableton, and loop only a small section (or only one cyle) in Simpler or Sampler, the "amp env pop" is still there. You can smooth it out with amp attack and release, but it's still present. This is related to the amp envelope of the synthesizer and/or sampler.
My workaround:
Resample or record the sine/wave sound from your synthesizer and loop the recorded audio clip somewhere in the middle at zero crossing points (do not crop the clip because you will get some "pop" again) - loop the clip - now you can play/trigger the clip without the "pop".
But as said, if you load that clip in Sampler or Simpler, you get the "pop" sound again.