We were somewhere around Barstow, on the edge of the desert, when the
drugs began to take hold. I remember saying
something like:

The flue goes up thru' a corrogated roof but I'm pretty sure that it
isn't the roof that's the problem. It really is the 200mm (or so)
diameter flue that's resonating. So it needs damping,

Are you sure the burner isn't pulsating? A common type of fault is
mis-matched flue and burner settings causing a pulsating flame and
sounds like a low rumble. Betcha the installer didn't set it up right.

Similar thing can happen if the burner is running on diesel instead of
kerosene - check the fuel.
--

Dave