it's the Hard Drive, sounds like the spindle or bearings are starting to make a noise after general wear and tear. If that is the case, when the noise is happening you might also be able to hear a "crunching" noise from the hard drive if you put your ear close to it when someting is being played off the Hard Drive ("crunching" is not a technical term, its just the closest word i can ever think of to describe it!)
Those are internal components of the Hard Drive, so they can't really be changed (its usually much easier to just replace the drive).
That doesn't mean the drive is about to fail imminently though, she should still be able to use it for a while.
A future planning question would be to think about whether she would want to keep the same DVR and replace the hard drive in future, or just by a newer DVR. The cheaper DVR's dont always come with instructions on how to replace and reformat the Hard Drive, but a quick google search of any model number should find that out