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Is there a dot in the magnifying glass in the upper right corner of your menu bar? If so, it's just Spotlight re-indexing the HDD.
 
Early 2008 MBP pro here and my disk has working constantly since install on launch day. iStatMenus indicates that it's writing at about 174K/sec CONSTANTLY. It's driving me crazy.

No reindexing. No obvious anything. Le sigh.
 
Same problem but can't say how bad it is compared to what you're experiencing. Nothing is running but there seems to be some activity.

Using iStat Menu, this is what I'm getting.
 

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