Smell from bathtub drain. Drain runs fine. I've cleaned with bleach several times, poured draino down, snaked it twice. I don't think there is any 'biofilm.' Smell remains. Smell not coming from anywhere else in house. Tub goes to old drum type trap. I wonder if the water in the trap is siphoning off because I can see periodic (~ once every few seconds) disturbances in the water level. The drum is under linoleum. Smell is worse after each recent use of tub. Thanks.