I have a British gas stove with the Euro modulating 630 gas valve. When it goes to the low side it starts to poof poof not real loud but you can hear it. I have cleaned the gas jet ( orifice by soaking in alcohol the blowing out with the air compressor out of stove. Cleaned the burner strip only has one. vacuumed the fire box out and reinstalled coals according to the manual. The gas pressure is 7 wc coming in Regulator is 3.5 wc to gas jet on high cold as spc. Gas jet is for this altitude been derated It just starting doing this poof poof this year. the air mixture is wide open for gas coal effect as spc, the flue pipe and air inlets are clean. This gas valve has a fixed metering set screw for the low side non adjustable The gas burner and manifold are clean no spider webs etc. Any body have any Idea as to the poof poof om the low side?