MacBook Air :: Hard Drive Going To Fail - Memory Available Keeps Changing By A Few Gigabytes

I have a 2011 MacBook Air and I love it but I am getting concerned that the hard drive is failing.  The amount available keeps changing by a few gigabytes..it will start the day with 10 or 11 GB available and then drop down to 9 or 10 GB and sometimes you can watch the amount available creep up in small increments every minute or so.  It is not constantly fluxuating and of course I understand that if I move files on and off the computer, the space available will change.  I have been using Macs for 20 years and I fear this is a sign my hard drive is going to fail. I am backing up the computer regularly.