Violin, Viola,Cello background noise

Can anyone identify the quiet, but obvious "shushing" sound found to some degree in all CD recordings of string section performances? It seems to be most apparent in sonatas and other chamber music. I can notice this sound with all my speaker sets, CD players, and amps, both tube and solid state. The sound is present to some degree from every CD manufacturer regardless of whether the recording is DDD or ADD.