So im just wondering if any ones got a suggestion as what this could be

whenever its raining or i just washed my car i get a nice and loud thump in the back suspension when i hit a bump. When everything drys up on the suspension it goes away.

 

can you duplicate the noise by jouncing on the rear bumper?

 

No only when its in motion

 

At this point I would check all the shock mounts and spring hanger bolts and bushes and sway bar mounts etc; at least for a start.

 

These units suffer from excessive rust issues in the rear suspension components-as well as the complete rear frame! A full inspection will determine if the unit is salvalgable.I retired a nice 96 due to this issue!