Interesting, as you say!
I used to think Grundfos were the best pumps going, wouldn't have anything else, but a doubt or 2 is creeping in. The original UPS 15-50/60 I believe was made for 30-odd years, but the UPS2 only 2-3 before being superseded by the UPS3, and I've heard odd reports about the UPS2, one being that if it is powered via the boiler (for overrun) it can fry the pcb. I fitted a UPS2 about this time last year (before hearing the rumours, and it isn't run via the boiler) and it's been OK, but there is a slight singing noise, maybe like the OP described. Unnoticeable during the day, but at night though it doesn't wake me, if I wake I can hear it. And it doesn't happen all the time. Had it on minimum fixed speed, and thought it would be better on PP - slightly lower flow. Set it on on minimum PP and it was worse, went back to fixed. It's nowhere near bad enough to warrant changing the pump, but it's made me think.
