I used to get this years ago (possibly when I started on thyroxin) but put it down to my neck being twisted as I slept on my stomach at that time. I stopped sleeping on my stomach and it went away but now its back since starting T3.
The only way I could describe it then was that it was like standing near a roadside when a huge motorbike roars past at speed. The sound and vibration is very loud and very fast and then its over. Now I am not experiencing the kind of roar I did years ago but it is the same sensation to a lesser extent. It feels like all the synapses in a pathway through my brain igniting at once then its over and, yes, there is a kind of sound involved although not as much as a fast moving motorcycle now. It is more of a hissing noise now I think. I'm not going mad this really is happening to me.