I’ve noticed the same thing on my TR2, and I think it indeed is the hard drive. I have a 2GB Toshiba PCMCIA hard drive that’s about 3 years old now, and it sounds the exact same way, only a lot louder.
Of course heat is one of the things that shortens a hard drive’s life span, and being located right next to the Pentium-M chip probably isn’t the best spot. Mine gets pretty toasty around the left palmrest area during normal usage. Let’s hope for a long service life on these hard drives!