Internal Alarm

Hi,

One of my computers a HP Celeron 500 is making an high pitch alarm
sound after about an hour or so of being turned on.

This first happened about a month ago, but the fan was making a racket
and on inspection, discovered that it had stopped working. I realised
that it was overheating so replaced the fan and the problem stopped.

However, it started happening again last night, but the fan seems
fine. Is there anything else that can be causing this problem.

I have two harddrives installed in my machine could this be causing
any overheating issues?

Any thoughts, gratefully received.

Hayley.