Both of these issues can be caused by cavitation in the pump. Cavitation is caused when the fluid traveling through the pump vaporizes (boils) because of the pressure drop going into the eye of the impeller. As cavitation takes place you hear little popping sounds as the air bubbles are created and then collapse within the different pressure zones of the impeller. The more air bubbles created the greater the impediment on the pumps ability to generate flow. Cavitation can also lead to pre-mature wear on the pump components. We want to avoid it at all costs.
Some of the factors effecting cavitation in pumps are; static head on the suction side of the pump, surface pressure, fluid, fluid temperature, and resistive losses on the supply side of the pump. Since your system design dictate your fluid, temperatures, and resistive losses many installers shoot to eliminate pump cavitation by increasing the static head on the supply side of the system by requiring the drainback reservoir (and thus the static head) to be 1 – 3 feet higher than the pump. While this is always recommended combining this solution with an increase in surface pressure is recommended as well. Wilo and Armstrong recommend a minimum inlet pressure of 14.5 psi for their pumps when pumping hot fluids.
So, the next time you install a drainback system bring your compressor and charge the system up to 14.5 psi to reduce the noise and reduce the chance for a call back from the pump not working.
To learn more see:
http://www.mcnallyinstitute.com/11-html/11-12.html