Trumpeting noise on Rinnai Zen

Absolutely stumped as to what could be causing this noise we are getting on every 10th or so Rinnai Zen boiler we install. Boilers make the noise on coaxial and atmospheric flue setups, cover on and cover off etc. Have checked the flue "sleeve" that has been known to drop down and we are almost certain the flue has no effect on the noise. Not even Rinnai can figure can give us an answer. Has anyone experienced this?