I made some new kits with op1 drum utility and when loaded my user samples all had clicks and pops.
Having just upgraded fw I downgraded, factory resetted and upgraded fw again.
Clicks and pops were still there. 
??? I was fearing an HW issue reading the posts here, but my metronome, factory samples and so on were sounding with no issues.
I then just used another USB port on my laptop and finally it worked as usual. The port giving me problems is one that charge even if the PC is shut down.
I hope this can help someone out in the future as It took me a while to figure out as I was sure to have already used that USB connection before. 
Hi! Although it was not possible to get to the true reason. But I think I’ve found a workaround. If you add a little effect on the problem channel, such as a reverb, the clicks disappear. I hope it helps.
Yesterday, my OP-Z began experiencing this issue. There’s a loud pitched tone when starting the unit, and then loud pops and clicks when playing a pattern. The clicks and pops continue for a few seconds after stopping playback. The problem occurs only when listening through the headphones output. USB audio is fine. A factory reset didn’t help. Fortunately, I still have one month of warranty remaining, and I have reached out to TE support. I am currently awaiting their response. Can I expect a prompt reply from them typically?
Still no news from TE support. Do you know how much time it takes them to usually reply?