A possible way for audacity and other sound register ...
For sure it's not a mathematical response, but I give you the solution
I've found because I already have this trouble of strange noise when I
record "directly" with audacity in concert or live session with music
instrument.
So I use a Sony MD player/recorder or a special music record external
hard drive to record concert session, and after I record from the
support to audacity.
I don't understand why, (the quality compression why not) but the noise
desapear and the record is good.