centrifugal pump

This is what cavitation sounds like

He's opening the valve on the discharge line from the pump which effectively reduces the back-pressure on the discharge side of the pump impeller. This means that the water/fluid is able to then flash off into a vapour far easier when leaving the pump housing. The noise you can hear at the entry to the heat exchanger downstream is the bubbles and water hammer effect on the plates inside the cooler. Using a discharge valve to restrict flow is not a great idea as it wears the seats and causes..

How to use a micrometer

Describes how to hold measure and read a micrometer