Wieght is a big factor. If you have a van with not enough weight for the suspension it will ride hard and it wil feel like you crashing into every bump in the road. If it's too light load it down with more weight and it will ride much smother. When it rides smoother there are less bumps to make noise.
Also check your tire pressure. Some auto manufactures put way too much pressure in the tires. I can't say if it is to help the new tire seat, to compensate if it does have a slow leak, or a tempiture thing. The dealer should have checked that with a new vehicle inspection but you never know. It should say in the door jamb recomended tire pressure. If you go on the lower side of those numbers it will ride smother.
Tom
