If it's "indexing", it matches the frequencies of the indicator flashes, if it's about 10K per quick flash.. 7 to 13 tiny quick flashes for every bright flash.. as does downloading pix files, but is about 20 times faster when the memory is processing other commands and processes...
If the system indexes at about 10K per second.. if it's indexing to index.. or if it's compressing data to quick-send, at start of boot, or at shut-down.. both processes sound the same from the hd...