Recently while making another batch of yogurt my PC screeched its complaint. After investigating and fiddling a bit, I decided that it happens only at high temperature. I pasteurize whole milk at 185ºF for three hours. A temporary fix was to remove the shield. I ferment the yogurt at 110ºF for eight or more hours as convenient. At this low temperature it has not screeched.
I just started fermenting another batch. Before this stem I dripped silicone oil down the impeller shaft and the little bearing(?) instantly felt better. It’s been about an hour now and all is quiet on the yogurt front.
I suspect the longevity of my PC due to ALWAYS having a cover on the bath keeping the high temperature and humidity away from the control head.