I had this sound too , brought it to SC and ask them to check few weeks ago, the technician said he couldn't hear anything -.-"
So I continue to drive normally , but until few days ago, the sound was miraculously gone.
The thing is I think the sound is not everytime there, i realize the sound is very obvious during hot weather and your car was running non stop for more than 6 hours (was doing grab car few weeks ago).