Before panic sets in,
all drives make a noise while they are reading/writing, and every OS - OSX, Windows, even my Linux-based network drive - accesses the drive regularly. The noise varies from drive to drive, so don't assume a noise means an ensuing problem. Like dtravis says, an actual drive error will probably affect the system in one way or another.
If you're concerned, keep an eye on the SMART status in Disk Utility, and also watch for the kind of pauses he described, including long delays while starting programs or booting up. A repetitive rattle/grind while opening a program or file is a good indication of an error - the system will retry a failed operation a set number of times. And keep your important data backed up, just in case!
