Computer sometimes detects HDD, sometimes doesn't

I have an AMD athalon 1GHz 1GB RAM desktop. It has a 20GB HDD. Recently, I booted it and it didn't detect the HDD and had boot failure. I tried to restart it a couple of times but it didn't start. So I thought, the HDD must have gone bad. One day later I turned it on and it booted up normally. I can't understand whats wrong. I didn't change any hardware and or install any new software. One thing I noticed is that the HDD makes a constant sound when the computer is ON. It seems like the niose a rotating part would make. It is difficult to describe but it is almost a hising sound. It is not a clunky sound. Any help would be appreciated.

Yea it definitely sounds like that drive is on its way out. First thing you might want to do is backup everything on the drive as a precaution, then run chkdsk /f to see if you have any bad sectors or anything. If it does have bad sectors then you will know for sure that the drive won't last too much longer. Good news is hard drives are pretty cheap these days, you can find 500GB for less than $100.