Navien NHB-055 Boiler rumbles while heating

I have a Navien NHB-055 boiler for the two-zone hydronic base board heat in my 900 square foot condo. It's a boiler only and does not provide domestic hot water. It was installed 7 years ago and only sees occasional use.

My issue: during the heating cycle, the fan speed diminishes until there's a loud rumble in the heat exchanger. Then the fan kicks on full throttle and later the system re-lights and continues to heat up to the maximum temperature.

Is this loud rumble a normal thing or could it point to an issue with the system?

I've attached a three-minute video illustrating this issue (apologies for the length - I've included timestamps so you can skip to the important parts).

0:00 - start of a heating cycle (flame and exhaust fan turn on)
0:55 - exhaust fan slows with flame on
1:53 - loud rumble and flame goes out (I notice some brief fire in the small heat exchanger window)
2:05 - exhaust fan slows and the system re-lights
3:00 - system reaches max temperature (188F) and finishes the heat cycle

For troubleshooting, I've had a technician from the installing company come to diagnose the issue where he:

• Cleaned the air intake trap (it was already 99% clean)
• Drained the condensate at the bottom (just a little bit of clear water)
• Cleaned the flame rod
• Checked the external air intake and exhaust were installed correctly and free of obstructions

But he was stumped as to why it was making that rumbling noise.

Any ideas why it makes this rumbling noise and is it something I should be concerned about?

https://reddit.com/link/1olxj09/video/ythith7nbpyf1/player