Oh man, this reminds me of my "party trick" back in the day of saying I could tell what OS a computer was running by listening to the HDD seeking. The good old days
Are you referring to something like the GPRS staccato coming from speakers catching a cell phone call or the almost imperceptible flyback whine of a CRT?
Linux ext* series - mostly silent, but even periods of high disk usage tend to be on the quieter side - probably due to lots of caching
MacOS - continuous, low-pitched "gritty" sound
Windows FAT - periods of silence punctuated by occasional intermittent groans
Windows NTFS - low rhythmic grunting, more continuous than FAT
Windows 9x - rather quiet, although periods of heavy activity can produce quite high-pitched seeking sounds