Hi all been to a worcester 42cdi (not my install) intermittent fault of loud vibrating suction noise same as if flue not connected properley has standard flue, problem worse case off! 9% co2 have checked intake 20.9 so connected ok ratio 0.0003 so nice and clean appears to have flame lift on burner at time of noise only 16mbar working at inlet test point and worcester spec 18 but have seen these run well at 14mbar before. and has been ok for past two years its a recent problem any ideas? occasionally flame will lift off that excessively flame failure code kicks in?