Explorer 8300 Question

The other day I went to a Cox Solution Store to swap a standard (non-HD) digital cable box that was acting up.  The rep gave me a Scientific Atlanta Explorer 8300.  I asked about the size of the box, since it was a lot larger than the one I returned (an SA 4240, I think).  He told me that these were now the standard boxes that were used for all applications (well, the non-HD ones anyway).

It seems this box has a hard drive in it and supports DVR functions, but my use of it is on a bedroom TV, so I do not subscribe to the DVR service for this particular unit.  Regardless, the unit seems to work fine as a straight cable box.  However, there is one perplexing thing...

I turn the cable box off when I am not watching TV.  When it is off, at exactly 3 minutes past the top of every hour, I can hear the hard drive spin up for just a couple of seconds.  And then it clicks off with no sound until 3 minutes past the next hour.  It seems to do this all through the day, not just in the overnight hours.

Anyone know why this happens?  Is this standard behavior for the 8300 unit?  If the box was in the living room, it would not be a big deal to me, but in the bedroom of a light sleeper, it is mildly annoying.