DJM 2000 pop/click sound when fx buttons are pressed

Hello friends,

I have question that I hope you will be able to help me with. I can hear a pop/clicl noise when I engage any of beat fxs or color fxs. The sound lasts only as split second, however, it is audible even when music is played and can even be heard in recordings.

Any ideas?

Thanks in advance,

Angel