Idk if it's a major thing tbh. Few people have reported the same issue here:
http://www.ign.com/boards/threads/whats-running-inside-ps4-when-you-shut-it-down.453520001/
And here is a video of the sound (not mine).
https://www.youtube.com/watch?v=hQcDFhLxckE&feature=player_embedded
sounds like a bee!! lucky mine hasnt had issues quiet as a mouse