It's kind of hard to tell without hearing it, perhaps you can make a video and post it on youtube or other hosting service for us to hear. But from your description I think I have the same thing happening now and then. I think it could be solenoid going bad, but it could also be weak voltage at the solenoid, or my battery going on 7 years old. I might trade the battery out of my wife's car and see if the problem goes away. If it doesn't go away then it would be time to measure the voltage going into the solenoid, and I may consider replacing the solenoid. I think I would rather replace the solenoid rather than the whole starter, because the starter is not giving me any other trouble.
If you have questionable battery terminals that could be the cause. A few years ago I had what I thought was a bad starter, but it was clamp-on repair type battery terminals that caused the problem. I hate those things, will never tolerate those again even for a quick fix. Don't go cheap on battery cables, and the top-post cables at auto parts stores are all crappy, so beware. Anyway I don't mean to go on and on...
If you have this type of terminal, change out your battery cables, not necessarily first thing, but soon:
