Recently solved a similar problem on our bike. Bearing in hub was loose as a result Avid disc would rub on brake shoes under pedaling presure. Noise was not always present because sometimes brake shoes would move far enough away from disc under factory installed spring presure other times the strength of the spring was not enough to overcome drag that is inherent in a long tandem braking cable.
I have also installed the TG recomended helper spring.The helper spring has benefits as the disc system gets dirty and does not move a freely as it did when the brake was new. I do not however like the change in feel at the brake lever so will be looking for a lighter helper spring.