Our home is 27 years old. A few months ago, a very heavy musty smell starting coming from our tub in the masterbath. Our home has 3 full baths and the problem only exists in the one tub. We have to keep a rubber stopper over the drain or it will knock you down. There is no problem of leaking water and a friend checked the vent coming out of the roof and it was clear. What could the problem be? Could it be something to do with the trap? Please help!