Violin, Viola,Cello background noise

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...