Buzz noise through venue PA

Hey. Running cello and pick up through berhnger istudio, Loopy HD- Audiobus-amplitude acoustic, out an amp to practice, behringer fcb 1010. There's a bit of buzz in the amp, not bad, but we did an advance sound check at the venue yesterday and it's pretty bad through the pa. Trouble shot a bunch and sound guy did great with it, but still there. Not grounding we think. It's an older ipad gen2 if that may be it. Would be nice to clear up. Any ideas?
Thanks,
E.