You know those few moments of warming-up noises the orchestra always makes–the tuning, the ‘white noise’ as the concert-goers find their seats–before the conductor walks on stage?
Where can I find a legal copy of that noise online?
You’re talking about two different things. The “white noise” part happens before all the musicians are on stage, and is basically individuals practicing some of the passages they’ll be playing. The “tuning” part* is done by the entire orchestra, usually led by the concertmaster.
*attributed, some say, to Beethoven 
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I have no idea if this is what you’re looking for, but the musical Chess has a brilliantly executed moment like this. It’s the intro to the song “Bangkok/One Night in Bangkok”, and it only lasts about 20 seconds or so. I recommend the London cast recording version; shouldn’t be too hard to find online.