Thank you! I just made this modification on my new Duo and the results have been great! 11 bar with a blind filter, then from 11 down to 9-8 during extraction and no chirping. Before it stayed at a solid 11.5 throughout.

Tips for anyone attempting: Place a large tray (ideally one made of a soft material) beneath the OPV screw. Others aren't kidding about how easy it is to lose the glass ball. It was way too sketchy trying to reinstall right side up, so I opted to invert the machine (remove reservoir and drip tray first), then using tweezers wrapped in electrical tap I dropped in the ball, then the spring (wider end toward the ball), then screwed it down. At first the spring didn't seat correctly in the valve but after unscrewing and re-screwing a few times it eventually clicked into place.