if you've got a vent to atmosphere DV on a BT escos, it will suck air all the time until some boost in produced and the diaphragm is forced shut from above. this is a big problem IMO, as unfiltered air can and does get sucked into the engine, hence why i removed mine altogehter and bunged it all up. just change the std air filter regularly as it offers superior filtering performance and achinves as good a air flow when its new/ nealr new as some panel filters which by virtue of their design will allow small particles of dirt through.