exactly bigdee, sewer gases are heavier than air, therefore , under normal circumstances there should be no odor from the vent pipe. but with wind blowing over the pipe ( which is probably being compounded by the shape of the op's home), there is a venturi being created, and the heavier than air gas is being sucked out of the vent, in exactly the same way as a carb works with a running engine as it pulls fuel from the float bowl. stop the verturi effect and the smell should go away.
