Fan runs at high speed for 20 minutes

I am having the same problem with my 1 year old Dimension E510.  No answers yet.

 

It's normal for the fan to 'jet' for 1-3 seconds at startup.  And if POST fails early, the high speed will continue until power is removed.  Fan will also run 'too fast' if the heatsink is misassembled (does happen, even from the factory), but it will continue running fast as long as it's on.

 

But both of these systems otherwise operate normally?  And the fan goes back to a normal speed after 20 minutes??  There's not a stock answer for that, that I know of (and I was current on cooling/fan issues up to a year ago).  It's a warranty issue if you're still covered.  If not, check that the heatsink/mount screws are secure.  But the 20-minute thing throws me, unless I'm misreading that.  The typical heatsink error results in normal startup then fan revving gradually to full speed during the first 2~10 minutes and staying that way--sometimes including overtemp shutdown.