clicking sound from left-rear wheel

I first noticed a sound coming from my left-rear wheel a couple of weeks ago, and since then the sound has progressively become worse.  The clicking sound was originally audible when turning to the left, at low speeds.  I can now hear the sound not only while turning at low speeds, but also while driving in a straight line at low speeds.  I also sometimes hear it when backing into my parking space.  

While the sound is mostly just annoying right now, I fear that it may become a much nigger problem if it is not addressed soon.  While I'm not above taking it to a service station, I would like to know what I"m in for before I take it to them (I'd like to know what the issue is before I go listen to them tell me it's something else - something much more expensive).