What would cause a rumbling sound when turning wheels at low speeds on 2004 Mercedes-Benz C240

Have 40000 miles on a MB C240 4matic. Just started noticing a rumbling noise when turning wheel at low speeds. Does not occur when turning wheel while vehicle is stopped. Do not notice when turning at speeds above 10 mph. Have been told this is normal with the all wheel drive over time. Would a transfer case fluid change help. Any other thoughts as to reason?

Asked by for the 2004 Mercedes-Benz C240