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. 