on occassion, the bathtub gurgles when I flush the toilet. And then on fewer occassions a sewer odor eminates from the tub right after it gurgles. The vent pipe is 2" diameter and sticks out the roof top by about 1 foot. Do I need a bigger and/or taller vent pipe? Could there be a bad joint somewhere? And once in the summer on a hot humid day there was a bad odor in the adjacent bedoom but NOT in the bathroom (go figure). Other upstairs bath has no problem at all. Will an air admittance valve help if used as a supplemental vent?