Sounds like the fan is going out. We have Dell Lattitude laptops at work. I've had to have the fans replaced in 4 or 5 of them. The first call to Dell, they were asking my what the fan sounded like. I told them like it was on it's death bed. About then the fan kicked on (or was trying too) so I held the phone so they could hear the noise. They immediately dispatched a service tech out to replace it. After that I didn't have any problems reporting the fans going bad.
They've been OK laptops otherwise, much better than the Gateways we had when I started. Some of them we've started replacing with IBM T30s.
If the fan is going out, it will sound bad sometimes, then quiet down, then flare up for awhile, then either stop completely or barely run, sounding like it is grinding.
Hopefully it's still under warranty.