Mildew smell from tub drain

The smell is not sewer gas it is more like the smell of a car carpet that has been wet . This is a tub shower with a 2 handle faucet . The drain has one of those toe tap plugs ( no lever ) . Started smelling this about 2 weeks ago and found it is coming from the tub drain . The drain has water in the trap and the smell continues even after using the tub .
Plugging the drain by pushing the toe tap down stops the smell . I hear no strange sounds or find the trap empty . The other tub in the house is about 8 feet away in the other bathroom with the same type drain setup and it does not smell . I pulled up carpet & find no wet or signs of it . smelling the floor/carpet gives no smell ( just sneezing ) The smell comes directly from that tub drain . The house is 5 years old and no hard use as im the only one who ever lived here .
Any ideas
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Check for hair under the plug, it should screw out and lift off. Dale

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Try a bio drain cleaner like Roto-Rooter Pipe Shield. I get the same thing from our shower.. It is just decay in the pipe and trap.
Randy
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Thanks i will clean it out .. funny how such a small area like that can smell so much .
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