My 95 deville rattles a lot when I hit bumps ,,,what could it be

When rattling u need to know where it is coming from, back? front? and closely pay attention. For a lot of cases like this u need to do investigation. if its front, look at outer tie rods and see if any are worn and loose which can cause wheel rattle, not often the case but it does happen in older vehicles. Look at lower ball joints and upper Strut mounts, the ball joints will pop in and out as u hit bumps and rough roads, the strut mounts like to wear out from the vibration and weathering, the insides are rubber and support around a disk-like bearing and deteriorate after long period of time. if its in the rear, check the strut mounts as well, they do the same as the front. check for rust all around the areas to make sure the metal has no rusted away front he strut mounts also which would cause rattling in the front. hope this helps and have an idea what to look for.

I would start with checking the muffler and pipes to see if they move when u push on them. Make sure car has set for a while so u don't get burned