Violin, Viola,Cello background noise

As a professional violinist, I'll add a couple things. The bow uses hair from the tail of a horse. This hair, especially when new, has microscopic barbs that hold the rosin. Rosining the bow enables the barbs to grab the string and the rest is obvious.

Depending on where the microphone is placed, string noise will be heard with some artists. Heifetz produced a lot of noise that could be heard in the front rows of the hall. This technique did produce an amazing, focused tone that soared over then entire orchestra and to the back of the hall. Heifetz got a bad rap for this, especially on many of his recordings. Those that heard him ‘live’ know/knew what an amazing violinist he was. I've been questioned for using this method at times in an opera orchestra, but ultimately I know something good and useful travels out of the pit. Fluffy playing is for the weak.

Its the artists and their clothing, or that's the trigger

only percussionist and the trombone players have to move as much as someone playing a bowed instrument.

What makes the sound strange is the gating and compression used in the recording to help reduce extraneous noises from creeping back onto the mics which could carry over onto oncoming parts of the performance and be even more distracting. Just shifting in the chair makes some noise that a mic will pick up. So gating is used and that's mostly what your're hearing.

At school we used to watch quartets in a church and the lite attendance for Friday evening concerts allowed us to sit right up front and the players clothing (especially men in tux/suits often made extra noise as did moving in their chairs.

All things you don't want on a recording.

I think this experience undelines some of the differences between live performances and recordings, especially if close micing is involved. If you listen to rock or jazz, think about how much more information you here when listening to an upright bass or drum kit through a high-end, high resolution system. Live sound and reproduced sound will probably always be to different, and potentially equally satisfying, experiences.