Recently, the bathtub in my daughter's bathroom started draining very, very slowly. I bought a bottle of Draino from the local store, followed the directions on the bottle, and after a hour I went to flush the tub. The water now drains faster than before, but as soon as you turn the water off and the tub starts to drain, I hear loud gurgling sounds from the drain as the water leaves. What could be causing this loud noise? My house sits on a slab foundation, so I can't check the pipes. Any ideas folks? FYI, the house was built back in 1996.