The noise at the back could be a very simple problem to fix. At the back of my vec i had strange creaking noises occuring, even when driving on flat roads and found it to be rubber stoppers that hold the parcel shelf in place. I made sure these were secure lined them with grease and after that no creaks! As for the noises at the front, it may be your dash creaking (quite common in vectra's). Try these out, if this doesn't resolve the problem it may be your springs, shocks or bushes.
i have the same on my omega going one way over a set of speed bumps near me but not if i come up the road and go over the other side of the speed bump. sounds like something is loose on mine sometimes, especially from the front