Racing thoughts are not just "thinking fast." They are thoughts that just won't be quiet, that can be in the background of other thoughts or take over your consciousness, that gallop around in the sufferer's head like a carousel gone out of control.
Components of racing thoughts can include music, snatches of conversation from movies or television or books, one's own voice or other voices repeating a phrase or sentences again and again, or even rhythms of pressure without any "sound" in the thought.
The phenomenon called racing thoughts is distinct from "hearing voices," which is a symptom of schizophrenia, severe mania or schizoaffective disorder. Racing thoughts can be a symptom of mania, hypomania or a mixed episode, but unlike some other symptoms of these moods, can also occur during depression or an anxiety disorder. Sometimes it is accompanied by a pounding heart or pounding pulses, including drumming in the ears.
"I am constantly talking in my head all the time except when I am sleeping. I am always thinking, thinking, thinking."
from Susie on our About Bipolar Disorder forum