This might not help but ... I encountered a similar problem and when talking with one friend in particular. Turns out he had a phone with the number pad located between the ear piece and the mic. He kept hitting his cheek on a digit (hence the loud tone). For years, he was in denial; he was too embarrassed to admit that it was him making the noise. Now I know the cause, the noise doesn't bug me so much.
Gato London
April 2006