By the way, I still don't quite understand how the 1.7ms gaps Don mentioned can cause the clicks/pops.
What exactly happens in these 1.7ms gaps? Is there no output at all? Or is the output frequency varying/unstable in that interval? I haven't measured with a scope yet, perhaps somebody already did?
Did the older library 2.0.1 not have any tuning gap? Or were they much shorter perhaps?
I believe most of the clicking was caused by noise coming in to the volume pot leads. The clicks could were heard equally loud even when the volume pot was turned to minimum position. Shielding those wires almost completely solved the problem in my case.
So it seems that the raduino generates this noise (which is then probably coupled via the pot leads to the input of the LM386), but I cannot see what the 1.7 ms tuning gaps could have to do with this (and the si5351 library version). Well apparently it does have an effect, but I just don't understand the mechansim behind this.
Perhaps somebody has an idea?
73, Allard PE1NWL