bathtub ordor

Every time I use my bathtub there is a foul smell that emits from the drain. Sort of rememnicent of cat pee. It hangs around for several hours and then it's gone. I have tried everything from bio-enzymatics to sulfuric acid even though the drain is not clogged.
Anyone have any ideas?
Thanks

Depending on the drain type and tub stopper, it could be a bunch of hair caught in the stopper. You may have to remove the overflow cover, and clean things out from there.