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.
It is probably in all recordings, you just notice it more when the music is quiet.

There is white noise added to most digital recordings called Dither. Dither is a form of noise, or 'erroneous' signal or data which is added to sample data for the purpose of minimizing quantization error. Dither is routinely used in processing of both digital audio and digital video data. Jitter is not synonymous with dither. Some CD players have adjustable dither.
Could it also possibly be the sounds of the bows on the strings (close-miking can reveal this, particularly with smaller ensembles)? I'm not really sure what the sound you're describing really is until I hear it.
I do believe if your system is "resolving" enough, this kind of noise could possibly just be the microphone picking up the acoustics (surrounding air mass) of the room in which the recording took place... ie: shushing noise...
I can list many examples but I am unsure which recordings I would have in common with you. The ECM recording of Schumann's 1st and 3rd string quartets
(2003) is a good example. The sound is apparent throughout the CD but is very noticable during the 1st 30 seconds.