What Causes Occasional Ringing In Ears?

I have a low humming in my ear and have seen 2 ENTS who have tested me and stated that I have normal hearing and no diseases. Yet, I get an occasional 5 second ring in either my left or right ear. It's followed by a full ear or deaf ear feeling then goes away. Even though I have passed all hearing tests I am very anxious about the transient ringing and am under serious stress.