Hi Guillaume,

Please contact support@eventideaudio.com. They can help troubleshoot the issue. That said, does this happen when the pedal is isolated from the rest of the board, meaning guitar > Blackhole > Amp? Running it mono or stereo, there should be no hiss. Make sure the back switches are set to mono and guitar level. Does this happen with every preset or just a single one? Have you tried powering it with the supplied factory adapter? Are you sure you are using In 1 and Out 1? I’ve seen cases of people mistakenly using Out 2 in a mono setup.