Quite often, analogue circuits can produce a clicking sound when triggered, even if all oscillators and noise generators are turned off. The nature of an analogue circuit means it is very unlikely to get the noise floor completely quiet, and this is shown when a note is triggered. This is expected behaviour of the synth and is not a fault.

Clicks like this can often be compromised using filters of rolling of the attach in the amplitude envelope, although again this is unlikely to completely get rid of the sound.