I would suggest checking the u-bolts to make sure they are tight. Loose u-bolts can make these types of sounds and will eventually lead to broken leafs in your springs. Is it equipped with a rear anti-sway bar? Bushings in anti-sway bars and shocks can also make these types of sounds. It could be something as simple as a loose shock bolt. I have had all of these issues contribute to noises and broken leaf springs.
Vince