main combi starts with a bang

[E4gas]

Some times my brand new main combi starts with a small bang ,

 

[SimonJohns]

Congrats. What are you looking for?

 

[E4gas]

That's what my customer just told me on the phone , when I commissioned it it was quiet when it fired up , now they are saying when it fires up it makes a banging noise ,

 

[SimonJohns]

Low working press
Blocked burner
Poor spark
Dirty fan
Obstructed flue

Where do you want me to stop. Lol. Loads of possibilities

 

[jtsplumbing]

If new boiler just check gas press , anything else call out Boiler Manu.!!