When we added rooftop solar heating we had to upgrade to a VS pump just for the noise. On low speed, the pump did not generate enough pressure to get the water up to the roof and enough flow for the SWG. On high speed it could do both of those things at the same time but was very noisy. Since the two-speed pump only had a manual switch to select speed, we went to a VS pump that could be controlled by the solar controller. VS pumps are also a bit quieter anyway.
Now the pump is on just the right speed, no faster than necessary, when the conditions are right for heating, but nearly silent at other times, unless you open the pump enclosure and stick your head inside.