My '03 3500 had the same noise -- sort of a soft whining or moaning at low speeds and occasionally at higher speeds -- it came and went. 1st time I took it to the dealer, it wouldn't make the noise. 2nd time I took it, it did. Dealer said the standard fix was to replace the rear driveshaft and U-joints, which they did under warranty. Haven't had the noise again, but it has only been about 4 weeks.