Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
CH Demand
Place Telegan Clamps on the Flow and Return pipes close to the Combustion Chamber
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

On/Off switch on.
Allow fan to run and boiler to fire and allow system to reach a reasonable operating temperature turn off demand.
Measure the temperature of the flow and return pipes ascertain temperature difference between the pipes.

Expected differential approx 11-20 °C.
Click on your observation:
1. DIFFERENTIAL OK NOISE DUE TO EXCESSIVE HEAT
2.DIFFERENTIAL NOT OK NOISE LOCAL BOILING
3.DIFFERENTIAL OK SYSTEM NOISE