Hi,
A week before i bought WD 2TB my passport model, when start copying i heard some sounds coming inside this hard drive already i copied 1.6tb of data am afraid about this issue.
You should try testing the drive.
There are some troubleshooting steps provided on the following links:
How to test a drive for problems using Data Lifeguard Diagnostics for Windows
http://wdc.custhelp.com/app/answers/detail/a_id/940
WD external hard drive makes a repeated clicking sound
