Belive it or not OS's do stuff even when you aren't, so it's perfectly normal, it's not normal if a) it's really loud, this would point to a crappy or dying HD or b) the sounds are too long (0.5 secs or more) this could mean some spyware or hidden process using your HD, on a side note, if the OS is Vista and it's freshly installed (or you have installed lots of programs and created lots of files in a short time) it could be the search function indexing your HD contents, this could also happn if your power options are set to indexing only when the PC is idle.
Anyway, it's most likely normal.