I agree with replacing the rubber, i.e. trailing arm bushings, sub frame bushings. In metal there is something called a resonant frequency or frequencie(s) in which loose metal will vibrate causing noise.
It could be the tires but I wouldn't think so at slow speed. The OE stated it occurred at 2 different slow speeds. I assume it smoothed out at higher speeds ! Chances are the rubber is old, loose, cracked causing the car to shack.