Hi Clemens and Max,
the background sound (a loud carrier with a lot of harmonics as you see in the previous picture) definitely is present in USB Host Mode. I confirm Tickle is in this mode because I receive MIDI notes and CC in Axoloti, because no change in freq is produced while turning the timbre knob and because Tickle initializes in the way it always does when connected to my laptop. The sound of the piezo is also in the output, but the SNR is something like 6dB.
I also used a ground loop isolator (https://www.amazon.com/AUKEY-Ground-Isolator-Systems-Included/dp/B01L1NP7YI) between tickle audio excitation out and axoloti in, with a mono cable, and the sound persists. I guess it is some funny effect of adding your hardware in serial to another one (it happens the same with Axoloti or with a normal audio mixer)
Do you also have this background sound in your Tickle or is it only mine?
It is totally ok, I am trying to explore this excitation signal as it would be cool and easy to use for making synths with external gear like Axoloti or OWL. Maybe this is a feature you could offer in the future: an output with the piezo dry signal.
best and thanks