Same problem here, it ruined an entire important rehearsal recording and weeks worth of setup and planning :-/
Anyway, here’s my setup:
1. uTrack-X32 firmware 3865, x32-Rack firmware 3.08
2. Mostly happens on 48 kHz either immediately or after a couple of minutes. It’s mostly audible when the channel signal reaches a certain threshold (above -30dbFS or so). After it starts, it seems to be persistent, even after a console power cycle. Switching the console to 44.1 kHz immediately solves it. Switching back to 48kHz immediately brings back the crackling.
3. Cannot test the stock card right now, sorry, but I might have the time later this week.
The effect is the same when recording to USB drive or when using the card as USB interface with a MacBook.
I captured and uploaded an example of how it sounds, maybe it helps: https://drive.google.com/open?id=1JFsKEn_8g_RwVJXFkpRMstR42yI8VTvE
Let me know if there’s anything else I can help you with in order to fix this issue.