on can produce a click in many plugins and there is no way to avoid it. In these cases you don't want to automate the
You have other options. The more obvious is to automate the
depth. Keep it at
0 when you don't want the effect, and increase it on the parts where you want the effect.
Another popular option is to use two tracks (two audio tracks or one audio track feeding an aux track through a bus), one with the effect and one without it. You switch among them using
mute automation and/or audio edits. This requires more work but gives you extra flexibility, as you can very easily treat the wet and dry portions separately. Effects can dramatically change a sound, so you might want the wet part to be louder/softer, or equalized differently, with different delay/reverb/compressor, or whatever. This would be very tedious and time consuming using only one track for both dry and wet.
If you think you are doing perfectly fine with the tremolo changes, use one track and automate the depth. If you think the wet part might need some adjustments to fit in the idea and mix, use one extra track for the wet portion.