Symptoms of a defective hdd

[Pranajit]

Hello,
Of late I am paranoid about my WD Caviar Blue 500 internal HDD.Every time, at the shut down process,it emits an hissing sound just like letting jet off.
Is it normal?

 

[sneak_stealth]

I can't say I've ever heard my Caviar Blue make that sound, if you can, would you get the sound recorded.