Rinnai combi boiler loud noise when firing

[ffmicu]

Hi, not sure if this is posted in the correct section. I have a Rinnai E110cn combi boiler (NG). Recently started making loud noise when igniting/firing. No issues with heat/DHW at the moment, no error codes. Just hoping someone can help tell me what's causing the noise and fluctuating flame. Thanks.

 

[Twowaxhack]

May be a bad gas modulating valve.

The unit has several on it. They open and close to adjust the flame in order to heat the water.

You’ll need service, I doubt Rinnai will troubleshoot that with you. You would need a digital manometer to even start.