In my opinion that is not a normal sound. It sounds like the drive is parking the head or the head is crashing. I had an old WD caviar drive that started doing something similar (although a higher pitch) and it eventually died not too soon after the sound started.
Does this sound go away if you put the drive under load, ie start a bunch of programs stored on the drive (or write/read a large file)? What are your power settings on the drive? It may be that it is going into/out of a low power state.
Alternatively, it could also be a warning sign and your drive may be ready to bite it. I would definitely back-up any important information on the drive asap.