I'm not really sure about why or how it helps..
I know US involves "sonic" vibrations of a sort so maybe it is just the internal warming and getting the cells moving & detoxing???

Had time this am to do a quick search about it.

[Physical Therapy Ultrasound
# Physical therapy ultrasound is recommended for both muscle and joint pain. It works via a variable intensity of sound waves "shot" at the body. The energy from these sound waves can penetrate as far as five centimeters, though the intensity of the waves (and thus effectiveness) decreases the further it penetrates. One chief benefit is that these waves cause microscopic air bubbles that seem to stimulate the parts of the cell membranes important in healing inflammation, thus helping alleviate both sore muscles and joint pain.]
http://www.ehow.com/how-does_5645109...und-work_.html


[Ultrasound — ultrasound uses high-frequency sound waves to reach sore muscles and other tissues that are over two inches below the surface of your skin. As the sound waves pass through your body they vibrate molecules, causing friction and warmth. This heating effect helps flush the sore area and brings in a new supply of blood that is rich in nutrients and oxygen.]
http://www.allaboutbackandneckpain.c...al-therapy.asp


When I was having spasms and the acute pain & symptoms My neck was very creaky and shoulders somewhat at times.
We call the creaky neck sound - the "rice crispies" sound... it that sound what yours is like?

Do you have times where you can hear a pulse sound inside your head? or a whooshing sound - that could be a bruit - usually the scalenes are tight & pressing or spasmed on the artery and so the pulse sound is vibrated thru so that's why/how you hear it.
Mine was so loud it would keep me awake at times.