@ThomasMvller and you’ve now got a message with some aid for your clicks. 
As for the sound engine - it rocks! But some things are very hard to deal with, and this was a worst case scenario. A very low Hz note with few harmonics in a steady repeating pattern with infinite sustain. Doesn’t get much worse than that for the poor buffers. You’re basically directly triggering a discontinuity in a continuous signal, which is what is causing clicking.
Luckily you can get around it in two quite different ways.
Either:
a) Set Amp Env Reset to OFF, and Operator Phase Reset to OFF for all.
or (if you want your timbre to be consistent per trig)
b) Assuming that we are triggering this tone each quarter note, set the note length to 3.88, landing on just before a whole quarter note and change the Amp envelope Release time to a low value (depends on the sounds content, but I’m getting good results with 5 - 13)
This will fade out the sound rapidly just before hitting the next note, which will remove the click and cause as little silence in the output as possible. Note that if you’re outputting a pure sinewave at very low Hz, some clicking will probably remain unless it is completely silent before the transition.
(But if you’re going for the sinewave approach you should use option a)
The reason as to why is that most (all??) hardware deals with things on a frame size basis as oppose to most DAWs and plugins that operate on a per sample basis. But that’s a bit besides the point, this can happen in any system - for example have a look at this visual representation of what is happening with the signal:
Ouch!
That’s not very nice. What we want to do instead is:
At the transition, ensuring that the quite brutal discontinuity - in this case a transition from non-zero to zero - doesn’t occur.
So as you can see, it’s not strange that it’s clicking. Most (good) plugins interpolate between samples (in envelopes for example) often avoiding issues such as these, but regardless the scenario is tough to handle.