We have a foul smell (sewage-type odor) that comes up from our shower drain after the shower has been running for 5-10 minutes. This only began about a week ago, and it only occurs after the shower has been running for at least several minutes. The city water department, which also maintains the sewers, has check the sewer main in the street and found nothing wrong, and they have suggetsed pouring liquid bleach down the drain. I'm hesitant to use bleach because our pipes are quite hold (house built in 1942). What could this be?