I just went to suzuki showroom this morning. The mechanic there also admitted that the rear drum rubbers are soft which is causing the sound, I asked to change the rubbers but he said that the other rubbers which they have, will also be soft like the present ones, so they might not help. He just tightened up the chain but the sound is still there (may be 20% less sound).
I am planning to buy the rubbers from open market and ask Suzuki guys to change them. May be the rubbers available outside will be little hard. Jadoon bhai, from where did you purchase the rubbers? Actually this sound is getting on my nerves now... I need to resolve it asap.