Hall of Fame 2 Mini audible click in sound


I have an audible click in the sound when I switch on or off the Hall of Fame 2 mini pedal.

I tried with different power sources (Boss, 1-Spot, Powerbank) and it is still there, even when the pedal is the only one in 

the chain. What can be the problem (software or hardware) ?

Thank you

Greetings from Belgium