OK, so this is weird.

I have 2 Fireface UFX units. One in the home studio, the other one used with my band for sequences on stage.

The home studio one had a broken optical input from the day I got it which I now need (ADAT), so it's gone for repair.

The live one has now replaced it but shows incomprehensible behavior.

After anywhere between 5 seconds and 3 minutes of power on, a huge distorted signal comes into all the analogue inputs. It's horribly loud even though the meters in total mix only go up to about 60% of the max volume.

It stops only when :
1. I switch the sampling frequency to 44.1 kHz. Anything higher than that triggers the issue.
2. I push the front button to switch the front display of the Fireface to anything but the main / meters screen.
3. I power off (duh).

This doesn't seem to depend on whatever is plugged into the unit.

Any idea what's causing this ? It's never happened to us  on stage, fortunately - we would have killed the audience, and / or the other way around - but then on stage we only ever use 44.1 Khz.

Any ideas ?...

Thanks in advance.