Well I notice this yesterday, after installing Watch Dogs. The PS4 was downloading update patches for the game and I noticed the humming.
I picked up the console and the noise stop which makes me think the the problem may be the vibration the console does when spinning the disc, the vibration on the platform the console is make the humming noise.
The sound is quite noticeable and loud but it stop minutes after, mind you I was not in the game, I was on the downloading screen.
Today I'm going to test the cleaning solution and see if it works.