I had a similar thing a few weeks ago. I thought it was my pedal which is about dead anyways so I didn't worry about it. A few days later my crank came way loose; I rounded it in the before I could stop about half a block later. Replaced the arm to hear the sound still there...turns out it's the BB which is completely effed. I was planning to replace it anyways, but still...not a cheap thing to now need to replace before I was planning.