Macbook pro retina 13" buzzing sound near thunderbolt ports

Just the other night, it was really quiet in my room, when I noticed a slight buzzing sound near the thunderbolt ports of my 2013 Retina Macbook Pro 13", like gear turning. The sound was very slight, I have to put my ear near the left ports to notice it. The noise is not annoying at all. In fact I could hardly hear it during the day. Could it be some problem with the ports? It is not a problem with the fan because the fan is on the right of the Macbook pro (I saw the picture of the inside on iFixit).