Well I don’t have a full build. My understanding of the Noctua upgrade is the fan speed is controlled by the potentiometer in the case and not based on IPMI temperature sensors. So, I’m a bit surprised they aren’t running at some constant speed.
I wonder if this has to do with the “minimum RPM” issue described here;
Although that post is before the Noctua upgrade was available, apparently if a SM board detects the fans are spinning too slow, it believes there to be a failure, so it ramps them up until that clears and the fans spin down again, which cause the “fault” again, over and over. Maybe you just need to adjust the knob a bit so the fans don’t fall below 500 RPM or so? Or try one of the other things mentioned in the thread. I don’t know why this would have just started though.
Maybe @rymandle05 has a better idea.