Make sure when the bolts are replaced that you ensure that the hanger bolt holes are not damaged. The bolts have splines near the head that engage the hangers. If the bolts have come loose they may have rotated or vibrated around and enlarged the holes enough that new splined bolts, or the old ones, will not engage the hanger material. Elongated holes and worn bolts usually follow close on the heels of damage like that and that can lead to complete failure of the parts.
Howard and Peggy
"Don't Panic"