Set Appliance State:

Mains Power Off
CH Demand
Temperature control set to maximum
Attach temperature clamps to flow and return pipes
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Allow boiler operate until noise begins.
Measure the temperature of the flow and return pipes as close to the boiler as possible and ascertain temperature difference between flow and return.
Expected differential approx 11-20 °C.