My home is about 7 years old. The toilet in the master bath is just on the other side of the wall from the tub. Two months or so ago we replaced the toilet guts and the toilet seems to work fine now. However, we now have a new problem: Every time the toilet is flushed, a gurgling noise is heard in the tub. Worse, after a few flushes, sewer smells come from the tub drain. Running water down the tub drain instantly removes the smell, but after a few flushes, it comes back again. What is happening and how do I fix it?