Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
CH Demand
Attach temperature clamps to the flow and return pipes as indicated by the red arrows
Gas On
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Allow fan to run and boiler to fire. Measure the temperature differential as the noise occurs or if continual noise after approximately 5 minutes.
Expected differential approx. 11-20 °C
Click on your observation:
DIFFERENTIAL OK NOISE DUE TO EXCESSIVE HEAT
DIFFERENTIAL NOT OK OR NOISE IS LOCAL BOILING
DIFFERENTIAL OK SYSTEM NOISE