This is similar to venting problems for heating appliances; although gas appliances have a vacuum break in the form of a draft diverter to prevent sucking a flame. The chimney code requires that a stack be a minimum height above a roof of 2 feet higher than the nearest point of a sloped roof that is 10 feet away from the vent to prevent pressure or vacuum. Your vent may be too low. The same rule seems to apply in the case of a sewer vent. Perhaps a Licensed Master Plumber can give another successful anecdote to solve the problem.