Normally I don't submit "me too" posts, but this may be of help...
I had a "chirping" sound a while back, and thought it was the hard drive too.
In my case however, it wasn't the hard drive. It turned out to be a dry bearing in the CPU fan. I had it replaced along with the heat sink for $12.50 at a local computer repair shop.
Edit:
A quick search found this interesting site:
http://www.dansdata.com/fanmaint.htm