Hard Drive Noise?

Lately everytime my computer is turned on the hard drive (I assume) makes this loud growling/humming noise. By the time win98se loads and the applications have loaded the growling fades away and doesn’t come back. It only does it on a cold boot, not on a restart. I’m always afraid to shut it down thinking that it’s going to explode next time I start it back up again. Afraid it might be dying…any ideas?