It's slowly dying. Hard Drives that make any kind of noise other than the normal clicking, like beeping, and especially rhythmic ones (like on a constant loop) is a very bad sign. Even if it's not having any outward issues as far as freezing or crashing, it's still sick. Backup your files. Don't use Window's HDD diagnostic utility. Seatools is good, but there are others you can run from the command line that are good. Either way, make boot disk from the Seatools .iso and boot from it. Follow the instructions and you'll be good to go.