Noisy startup but good combustion numbers (mod/con boiler)

All,

I have a Peerless PF-80 boiler installed & on its third season (Southern Ontario, so from Oct-May). This season the boiler has started vibrating / making noise at startup, with severity ranging from a quiet normal startup to being loud enough to wake me up at night across the house.

At first glance it sounds pretty obviously like bad / delayed combustion, but I've had our hydronics guy back to analyze combustion & the numbers are perfect. We also tore the HX down and it looks in great shape internally. Some notes:

* The vibration occurs with varying severity, from totally normal startup about 1/3rd of the time through to being audible across the house about 1/3rd of the time.
* The noise sounds not unlike the vibration of dragging a chair across a wooden floor. mp3 file is attached.
* The noise happens only at ignition; the boiler runs normally once lit.
* Combustion numbers are bang-on, both high fire and low fire.
* Tear-down of the HX reveals nothing out of the ordinary.
* Occurs with and without the case opened, so air availability / temperature is likely not an issue (the outdoor intake vents directly into the enclosure).
* Outside / system temperatures don't seem to affect the likelihood of the noise, nor its severity.
* Gas supply line is new & well sized (3/4" CSST).
* The delay from spark to vibration is pretty constant, so I'm not convinced it's delayed combustion either
* My hydronics guy isn't super familiar with Peerless as a brand, so we're both a bit blind to anything specific there

We are both at a loss as to the possible cause of this. If anyone has any firsthand experience with this, or ideas about what to look at next, I'm all ears.

Thanks!