Our boiler (Baxi Megaflo System 32 HE) has, since new, been prone tovibration at certain levels of demand.  The oscillation is at quite ahigh frequency, I would estimate 15-20 Hz, and doesn't have a mechanical source (like the fan or pump) but seems to be a rapid modulation of the burner.  When it's happening, the flue gases have a distinctly different smell from usual, presumably because of ncomplete combustion.
t feels to me like a control-system instability, just like an nsufficiently-damped negative-feedback loop.  Changing the demand.(e.g. opening the valve to the radiator circuit) will usually supress it, after a few seconds of globe valve, and as the oscillation gradually dies down the frequency increases.  The boiler service engineer hasn't really a clue, especially as we've never managed to provoke it to happen when he's been here. My Keston did that when new, at low modulation levels.
It supposedly came with the mixture pre-set to the required value.On checking, it was miles out. Once set correctly, this stopped.Has the service engineer put a flue gas analyser on it to check the mixture at all modulation levels (at least, both max and min)?Some people with the problem on the Kestons got Keston themselves
out, and they fitted a restrictor on the air intake inside theboiler. That's one strictly for the manufacturer to do though, or he boiler's gas certificate is voided. In my case, correcting the mixture fixed it.In the Keston, the modulation is done by varying the fan speed.The gas rate is controlled by the pressure at the gas mixer
venturi (which is in turn a function of the fan speed). It's not difficult to see how you get a positive feedback loop here,with a time constant of the order of the response time of the pressure reducing valve (which is what adjusts the gas flow rate),which is very likely 15-20 Hz. On mine, it sounded like a 32' organ pipe.

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