Subject: Why do power lines make a crackling sound?

A friend of mine has a high power electricity line going by her
apartment.  The lines make this crackling sound, line static, but
it is not steady at 60 Hertz, but rather sounds like white noise.
It seems that the noise could not be due to dielectric  breakdown,
of eventually the wire would be shorted out.
Can anyone tell us what physical process causes this noise?

Thanks,
Daniel Williams

Re: Why do power lines make a crackling sound?