Could be the sway bar (bushings?)... mine recently devoloped a noise when driving slowly over bumps but it only does it when there are four people in it. Hopefully someone that knows what it is for sure and how to fix it will reply.

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1985 300D Turbo ~225k
2000 F350 (Powerstroke) 4X4, SWB, CC, SRW, 6spd ~148k
1999 International 4900, DT466e (250hp/660 ft/lbs), Allison MD3060 ~73k