I bought an X32 rack last month. Updated the firmware to v4 and installed the Edit software. All good. But whenever I playback from either my iMac or Macbook Pro (both running Pro Tools and iTunes on Mojave) I get intermittent crackling and pops via the monitors (and visible on the x32's metering) similar to sample rate mismatch.

I bought a UPS to eliminate any potential bad power causes, but the issue is still there. The cracks and pops hit close to digital zero, regardless of playback level. 

I have set the Aux to 'Card 1-2' in the routing to facilitate playback via USB.

Any help would be appreciated as I've been trying to diagnose this for over a month and am at the point of just returning it for a refund!