[Elecraft] K3 Audio instability

[Mark Pride]

 Recently noticed a problem with receive audio.  Hear a crackling sound while using the headphones and that noise does NOT vary as a function of the AF gain control.  It comes and goes, very random and very objectionable.  Almost like hearing instability in the audio stage of an old vacuum tube receiver - if I can coin it that way?

Suggestions for the cause and fix?

Regards,
Mark, K1RX