Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
CH Demand
Attach temperature probes to the flow and return pipes as close to the boiler as possible
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Allow the boiler to fire for Heating and once it has reached a reasonable operating temperature turn off the demand.
Using temperature probes and Performance Tester (FGA) measure the
temperature of the flow and return pipes as close to the boiler as
possible and ascertain temperature difference between flow and return.
Expect a differential of approx 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