Elektor Formant Modular

R-C is a resistor-capacitor combination.
This creates a sort of slow on-off for an LFO.
You can also think of it as a low-pass filter. The edges of a square wave
represent a high frequency, wich can be filtered out with an R-C network.
same effect if you would modulate a VCO with an ADSR with fast attack and
release times, you would also hear the clicks.

greetings !