Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
CH Demand
Attach temperature probes 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:

With the burner lit and established measure the temperature differential when the noise is present.
Expected differential approximately 11-20°C