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Lochinvar whn 85 fog horning

I am looking for some help in solving a "fog horning" problem with a lochinvar whn85. This boiler will fog horn from 10 seconds to about a minute when it modulates down to 20%. I have tried to tune out the noise with venturi and gas valve adjustments using a combustion analyzer with no results. The boiler rep came down and also tried to tune out the noise with no results. The install is done according to the manufactures instructions and the vent and air intake are within limits. The only way to make it stop is to remove the air intake at the rubber coupling. I don't want to use room air as it is a direct vent setup. Any help or info would be greatly appreciated. Thanks, Jim.