Do you know why an unpleasant odor comes from my bathtub drain?

There is no hair or anything gross in the drain and it runs perfectly well so there is no blockage. We clean them often. The odor isn't all the time, just now and then but it can last several days. Sometimes running water down the drain gets rid of it but not always. We are on a septic system but have had no problem with slow running drains.